This seems like a very easy grammatical aspect but a lot of people have trouble with it. When speaking both these words sound the same which is why it can be very tricky when writing and using these. The best way to learn how to tell the difference is by understanding what they mean. 'Est' can be translated as 'is' and is a form of the french verb 'Être' (which means to be). We would use 'Est' in the third person singular, e.g.; il/elle/on. An example of this would be 'il est Anglais (he is English). However, 'Et' is used as another way to say 'And'. An example of this would be 'Mes chaussures et mon pull' (My shoes and my jumper). As you can probably hear they sound exactly the same orally but written down they mean different things. When writing in an exam, these little aspects of your writing are very important. Once you can master the basics, the more complicated tenses and conjugations will be much easier to understand.