How do I write a good essay in a foreign language?

This is something that can make the difference between a C and an A at A Level for languages. Speaking from my experience of Spanish A Level, there are a couple of things that you can do to up your writing game.

1. Speed up. I know this sounds counterintuitive, but if you spend too long thinking about what words you're choosing or trying to use a particular structure (e.g. the subjunctive), you will lose what matters most: what you're actually trying to say. You can lose yourself and the semantics of the sentence very easily if you spend too much time staring at one line or one phrase. Just concentrate on getting what you want to get down on paper, and when you feel more comfortable, and start to gain greater written fluency, then you can take more time over it. Remember - you get more credit for what you say and how much it makes sense than how impressive your structures are in an A Level languages essay.

2. Learn to understand sentence structure. When you understand what you can do with a sentence, how they work in the foreign language you're learning (syntax works differently in different languages!) and what sounds good to a native, your essay writing will improve enormously. Learn how subordinate and coordinate clauses work in the target language. For Spanish particularly, a good understanding of relative clauses and relative pronouns is exceedingly useful. 

3. Keep it brief. Most language essays will have a tight word limit - mine were 400 words (+/- 10%) - so don't repeat yourself! Your examiner knows what a good point is, so don't state it at the start AND at the end of a paragraph. This goes for the introduction and the conclusion too - they aren't for merely talking about what you've written. Think about it: if that were their true purpose, their existence would be irrelevant. Instead, try to use them wisely. Use the introduction to make you sound clever - talk about the topic or title; discuss its nuances or define a word that might cause confusion. Use the conclusion to explain how and why you've come to the viewpoint you've come to in the essay. Coming down hard on one side of the argument can help you focus your argument and thus your conclusion, but if you're intelligent about it, you can sit on the fence in the conclusion and still write a great essay.