The most common mistake in an open-ended exam is knowing the answer and writing it badly. A student who finds the right result and puts a single number under the question can score lower than one who half-remembers the topic but lays out the method. Marks follow the mark scheme, and mark schemes are built out of steps.
The good news is that this is a narrow, learnable skill. Unlike the content itself, it can be fixed in two weeks.
How are open-ended answers marked?
Against a scheme that breaks the full answer into parts and attaches points to each: picking the right data, setting up the relation, carrying out the operation, the unit, the justification. The person marking is looking for those parts. The question is not "right or wrong" but "how many parts are present".
Two things follow. Partial credit is real, so every correct step earns something even if you never reach the end. And a step you skipped counts as a step you do not know. Reasoning you held in your head is invisible to the marker.
Where does an answer start?
With the verb in the question. Open-ended questions usually carry one instruction: calculate, explain, compare, justify, give an example, interpret. These ask for different shapes of answer and they do not substitute for one another. Writing two definitions for a question that says "compare" misses the comparison marks; the similarity and the difference have to be stated.
A habit that helps: after reading the question, note two words in the margin, the verb and the quantity. "Explain, 2 reasons." "Calculate, unit needed." That note keeps the answer on track while you write it.
In maths and science, separate out what you were given. Writing what each number is next to it stops you getting lost in a long scenario. A long question is not automatically a hard one; usually one thing is wanted and the rest is the story around it.
How long should the answer be?
As long as the mark allocation. Half a page for a five-point question wins nothing and costs time; two sentences for a ten-point question leaves most of the scheme untouched. A rough measure is one short sentence or one line of working per point.
Writing too much carries its own risk. The scheme hunts for correct parts, but a marker who sees a clear factual error reads that too. An extra claim you are unsure about does not strengthen an answer.
How do you show working in maths?
Line by line, aligned on the equals sign. First line, the relation or formula. Middle lines, the numbers substituted in. Last line, the result with its unit. Even steps you did mentally deserve a line, because that middle line is often exactly where the points are.
Three losses show up again and again. Leaving out units, where 24 and 24 cm are a point apart. Stopping at an intermediate value instead of the one asked for, like finding a side length when the question wanted the area. And solving in the rough-work space, then copying only the result across, which means the work you did is never read.
When you cannot finish, keep writing anyway. Listing what you were given, naming the relation you would need and showing where you got stuck scores better than blank space.
What about essay-style subjects?
Claim, evidence, link. State the judgement the question asks for in a clear sentence, support it with something from the text or from the topic, then say in one sentence how the two connect. Most literature, history and geography questions are hunting for those three pieces.
In text-based questions the evidence almost always has to come from the text. An answer that gives your interpretation without quoting a single phrase stalls at half marks. A short quotation makes the evidence visible.
One more rule holds in language subjects: give the number of things asked for. Three reasons for a question that wants two earns nothing extra, and if the third is wrong it can cost you.
Leave it blank or write something?
Write something. Nothing is deducted for a wrong answer, so a blank line has no upside at all. Even on a question you cannot do, you can define the key term and say how it relates to the topic, and most schemes keep a point for the definition.
How do you split the time?
By marks, not by question count. Look at the point values before you start and mark the heavy questions, so you are not writing the most valuable answer in the last ten minutes. If the paper runs forty minutes, the average question gets under five, which is no time to build a plan mid-exam.
Start with the question you know best. It buys an early mark and it steadies your handwriting, and handwriting matters here: an answer nobody can read cannot be marked. If nerves are the part that undoes you, what actually helps with exam anxiety and managing time in an exam go further on both.
What should you practise beforehand?
Solving with the answer written out. Take five questions, ignore any options, solve them and write the answer in full sentences. Then read your own page as a marker would: how many parts are here, which one is missing.
Twice a week is usually enough. Any practice set works, including our reasoning tests for the thinking side; what matters is that you write the steps instead of recognising an option. The skill grows faster than subject knowledge does, but it still does not grow overnight.