The Grammar Errors That Actually Cost Marks
Readability weighs errors rather than counting them. Ten harmless slips cost less than two that make a sentence ambiguous, and knowing the difference changes what you check.
Errors are weighed, not counted
Readability is 25 per cent of the writing score and covers grammar, sentence variety, spelling, punctuation and paragraphing. The decisive phrase in the criterion is whether errors hinder understanding, and frequent errors that affect meaning are what reduce the score.
This reframes the whole problem. You are not being asked to write without mistakes. You are being asked to write so that a reader never has to stop and work out what you meant. Those are different targets, and the second one is achievable in three minutes of checking while the first is not.
It also means the errors worth hunting are not the ones you were taught to worry about. Articles are the most corrected feature in most English classrooms and among the least costly here. Tense consistency is rarely drilled and is one of the most costly.
The errors that cost the most
Tense shifts mid-narrative. I called your office on Tuesday and I am waiting since then, so yesterday I try again. Three tenses, and the reader has to reconstruct the sequence. The fix is to decide your timeline before the paragraph: past events in the past simple, states continuing to now in the present perfect. I called your office on Tuesday and have heard nothing since, so I tried again yesterday.
Subject and verb disagreement across a long subject. The box of kitchen items that were delivered on Friday were missing several pieces. The subject is box, singular. The box of kitchen items delivered on Friday was missing several pieces. This error is expensive because it usually appears in your most ambitious sentence, where the reader is already working hardest.
Run-on sentences joined by commas. I unpacked the boxes the cabinet was cracked I called the office twice nobody answered. Meaning survives but the reader is doing your punctuation for you. Split it, or join with and, but, so.
Conditional and time-clause errors. If you will send a technician, I will be at home and When I will arrive, I call you. Both force a re-read. English puts the present into the clause: If you send a technician, I will be at home.
Dangling or misplaced modifiers. Unpacking the boxes, the cabinet was cracked says the cabinet was unpacking. When I unpacked the boxes, I found the cabinet cracked.
The errors that cost the least
This list is not permission. It is triage, for the three minutes when you cannot fix everything.
Missing or extra articles. I sent email last week and I went to the university on Tuesday. Both are visible and neither obscures meaning. In a response that is otherwise controlled, article slips cost very little.
Preposition choice in fixed phrases. on the weekend against at the weekend, different to against different from. Regional variation covers much of this and the rest is rarely ambiguous.
Plural of an uncountable noun. I received many informations is wrong and reads as a learner sentence, but no reader is confused.
A single spelling slip in a long word. accomodation, recieve, seperate. Worth fixing if you see them, not worth hunting at the expense of a tense error.
The ordering principle is one question: would a reader have to stop. If the answer is no, the error is a low priority whatever your teacher said about it.
Sentence variety, which also lives here
Variety is named in the criterion and is the part candidates most often neglect, partly because it is not an error. A response of twelve sentences all fourteen words long, all built subject-verb-object, reads as flat even when every sentence is correct.
Variety has two cheap sources. The first is length contrast: put one short sentence next to a longer one. I called your office twice. Nobody answered, and the second call was cut off after eleven minutes on hold. The short sentence gains force from the long one beside it.
The second is opening variation. Not every sentence has to start with the subject. When I unpacked the following morning, I found the cabinet cracked. Because the box has not arrived, I have been buying replacements. Unfortunately, two calls have gone unanswered. Three different openings, no extra words.
The caution is the same as everywhere else in this section. If a longer construction is going to break, write the short one. Variety earned at the cost of a dangling modifier is a net loss, because the modifier error sits in the expensive column and flatness sits in the cheap one.
Read only the verbs on your first checking pass. Tense drift and subject-verb disagreement are the two errors most likely to make a reader stop, and both are invisible when you read for general sense.
Punctuation and paragraphing
Punctuation matters here mostly where it changes grouping. A comma splice joining two full sentences is the common one, and the fix is a full stop or a conjunction.
Apostrophes are worth a moment because the error is visible and easy. its is possessive, it's is it is, and in a formal email you should not be writing it's at all, which conveniently removes half the risk.
Capitals after a full stop, and a full stop at the end of the last sentence, are the kind of thing that gets lost when the timer is at thirty seconds. They are also the fastest thing to scan for.
Paragraphing is explicitly part of this criterion, so a 180-word block of unbroken text loses marks here as well as under coherence. Three or four paragraphs of three or four sentences each is the target, with a blank line between them so the structure is visible on screen.
There is no spellchecker and no autocorrect in the answer box, so anything your phone normally repairs will stand. Words you habitually mistype are worth knowing about in advance.
A three-pass check that fits in the time
Do not read for everything at once. Three fast passes catch more than one slow one, because each pass looks for one thing.
Pass one, verbs. Read only the verbs. Is the timeline consistent, and does each verb agree with its subject. This pass catches the two most expensive error types and takes about forty seconds.
Pass two, sentence boundaries. Read for full stops. Any sentence over about twenty-five words is a candidate for splitting, and any comma joining two complete sentences needs to become a full stop.
Pass three, surface. Capitals, apostrophes, the spellings you know you get wrong, and the greeting and sign-off. Thirty seconds.
If a pass finds something you are not sure how to fix, replace the sentence with a simpler one rather than repairing it. Readability rewards control, and a plain correct sentence always beats an ambitious repaired one.
Any sentence longer than about twenty-five words should be split unless you are certain of it. Readability scores control, so two correct short sentences beat one long sentence with a broken clause.
Practise it
Five pairs to judge. In each, decide which error a reader would actually have to stop for.