1 Ambiguity

You thought you said what you meant but in fact the sentence can be understood in two different ways. This is a common crime against clear writing and has many different causes. But I'm not going to make it easy for you. Can you spot the problem and its cause in each of the sentences below?

All politicians are not liars.

She only smacks her children if they are naughty.

A selection of second-hand books is for sale in this shop in pristine condition.

Refunds will be given on faulty goods with no strings attached.

Driving through the fog the road seemed to disappear into the gloom.

Males will be eliminated in this study.

(Explanations: Better to say, 'Not all politicians are liars' as the first version implies that no politicians are liars.
Put 'only' before the word it modifies: 'She smacks her children only when they are naughty. The first version makes it seem as if she could do worse than merely smack them.
Reorder the sentence because it is the books, not the shop, that are in pristine condition.
I don't think the fault lies in goods having no strings.
The dangling participle makes it sound as if the road is doing the driving.
I don't think the outcome of the study is intended to be the elimination of males; rewrite: males will not be used in this study.)

2 Vagueness

Okay, so vagueness is not always a sin. How so? Often the writer uses vagueness to avoid a factual error: vagueness is a way of admitting we don't know it all. When we write 'approximately six months', 'in about a year', 'after a few days', we may deliberately want to leave the precise times unstated to give some leeway. This kind of vagueness can be irritating if overused but it can sometimes be justified.

However, vagueness is often used to disguise the truth, to mislead or simply to be unhelpful and then it is indeed a sin. Vagueness about time spans could be a sin if the whole purpose of a document were to provide time-sensitive information. Thus, if we read: 'it takes approximately six weeks to process an a application', this is unhelpful. It would be better to set a time limit: 'applications are processed within six weeks.' If such a promise cannot be made then an interim measure is needed: 'applications are processed within six weeks. However, sometimes delays occur. If you have not had reply after six weeks, please telephone and we will give you a report on the status of your application.'

In scientific documents, vagueness can detract from credibility. After all, science is supposed to be precise. Rather than saying 'about 60 percent', 'approximately 60 percent' come clean about the situation. If the figure hasn't been measured precisely it is an estimate, so say 'an estimated 60 percent'.

Sometimes vagueness is used to soften the message when something harsh or embarrassing needs to be expressed. I heard a physician talking about people being 'in a state of negative existence', but I'm not sure this makes me feel any better than hearing that they are dead.
So, whenever you are tempted to be vague, think about the context and predict what effect it will have on readers and avoid it if possible.

3 Padding

This is the most frequently committed sin of all. Keep it lean. Padding wastes time and makes the message fuzzy. Look at this sentence:

This chapter provides a summary review of material outlined in the preceding chapter and gives a general overview of the main causal factors underlying those events.

I'm not joking; I meet this verbiage all the time. Rewritten is boils down to:

This chapter examines the causes of the events described in the previous one.

The most frequent culprits are adjectives and adverbs. People sprinkle these around like pepper and salt but they should be saved to add meaning rather than flavour. Sir Ernest Gowers in his classic and unsurpassed The Complete Plain Words, warns against some of the worst offenders. Among these 'respectively' stands out. He quips:

' "Men and women wear trousers and skirts and knickers respectively." Who wears the knickers?'

Other contenders for being high in frequency but low on meaning are: definitely, inevitably, actually....I'm sure you can think of plenty more. And have you noticed how uppity the preposition 'up' has become: listen up, park up. It is quite sufficient to listen and park.

Sequencing phrases can also be offenders and often expand beyond their required line space: 'First of all', and even 'second of all', are no better than 'first' and 'second'.

A good exercise when you have written a paragraph or two is go back and see how many words you can take out without affecting the meaning. It's instructive how much you can take away.

4. Clichés

'At the end of the day, when children come into school in the morning....'
Please, Ms Morris, think before you speak.

In writing, the use of clichés is the lazy way out; they are definitely the thin end of the wedge and in good writing are conspicuous by their absence - see what I mean; they slip in so easily.
AVOID at all costs.

5. Jargon

AGP Aperture Size: this option determines the effective size of the graphics aperture used in the particular PAC configuration. The AGP aperture is memory-mapped, while graphics data structure can reside in a graphics aperture. The aperture range should be programmed as not cacheable in the cable cache, accesses with the aperture range are forwarded to the main memory....

I hope you feel better for knowing all that; I wonder what it's all about. If you respect your readers you will find ways of expressing technical data comprehensibly. Remember too that we live in a visual era. Sometimes a diagram, graph, table or chart can do the job much better than words. The main concern of the writer is clarity and if it takes visual support to achieve that, then all well and good.

6. Incorrect Usage

I'm not a prescriptive grammarian. I recognise that if languages stop changing they must be dead. It's easy to lay down the law about Latin grammar because it's fossilised and nobody speaks it. However, a writer needs to know how the language has evolved so that the decisions made about usage are informed choices and not simply a question of following the latest trend. I am not in favour, for example, of using prepositions with a nominative case simply because a lot of people no longer know prepositions take the accusative. Hence, for me, 'between you and I' is a language sin because it arises from ignorance.p>

However, on the topic of 'less' and 'fewer' I feel the battle has probably been lost. Fewer people than ever use these two correctly so I feel I want to put less effort into persuading other writers to preserve the distinction, although I shall continue to do so. The same is probably true of the distinction between 'who' and 'whom'. However, the difference between 'which' and 'that' needs care as it can alter meaning.
Take these two examples:

Sweets, which are bad for children's teeth, should be avoided.
(All sweets are bad for children's teeth).

Sweets that are bad for children's teeth should be avoided.
(Avoid only those sweets that are bad for children's teeth).

Care also needs to be used with conjunctions. Avoid yoking disparate ideas together with 'and'. 'And' indicates sequential ideas but the sequence should be logical. In the example below it just doesn't work:

She works in a bank and her family takes holidays in August.

The two ideas have no logical connection.

Ultimately correct usage is an aid to clarity of expression and that is why the writer needs to take care with it.

Of course one of the issues with English now is that, as a global language, it is subject to many more and varied influences than ever before. It is thus entirely reasonable to accept that different varieties of English have different conventions. Again, the writer needs to be informed. In American usage it is acceptable to say 'I just went to the bank'; in British English it is not; British English still uses the present perfect tense: 'I have just been to the bank.' For the writer it is with usage as with the other issues raised here a question of respect for the readership. Use those conventions of usage that your readers expect to find. But take particular care with subject/verb agreement. 'Data', for example, is a plural form, so make sure you give it a plural verb 'the data are' , not the 'data is'. Composite subjects are trickier. If you feel that the subject of the verb should be taken together, as in ' a large number of experiments', then use a plural verb even though 'number' is a singular noun.

This brings me on to another problem though. As with this article, much writing is aimed at an international readership. So which conventions to use? I'd say, use your own variety of English unless an editor requests something different. Here I've used British grammatical conventions and British spelling, but would find it entirely reasonable to change it to another variety if required to do so.

7. Commonly confused words

It would take a voluminous document to deal with all the potentially easily confused words in English. But here are just a few of the more common offenders:

childish/childlike

childish is pejorative and applied to adults when they behave in a way more suitable for children:

his interest in trains seems a little childish to me

childlike means innocently simple like a child:

she retains her childlike enthusiasm for exploring new places.

to clean/to cleanse

to clean is the verb used generally for the activity of making things clean:

you will have to clean all this mud from the floor

to cleanse suggests the action of cleaning makes something pure or sterile:

cleanse the skin thoroughly every night
cleanse us of all sin

continual/continuous

continual refers to something that happens often but with intervals:

I am tired of the continual noise from my neighbours

continuous refers to something that happens without interruption:

a continuous buzzing in the ear is a symptom you should have checked by your doctor.

credible/creditable

credible means something that can be believed:

his story of how he damaged the car was entirely credible

creditable means deserving praise:

he made a creditable effort to pass his exams.

deadly/deathly

deadly means something that could led to death:

the poison is deadly and there is no antidote

deathly means as quiet or as pale as the dead:

there was a deathly silence in the crowded room.

So, if in doubt please use a dictionary!.