Science demands precision, and part of mastering a subject involves learning the exact differences between words that mean more or less the same thing. An entomologist, for example, knows the difference between a grub and a maggot, whereas for the rest, they are simply worms. However, to a software engineer, these words mean something completely different. At times, the only difference between two words is that one is more common in Britain and the other is more common in the United States: a lift and an elevator, for instance, or a queue and a line.
English has a very large vocabulary. However, to write well, it is not enough to know many words; rather, it is more important to know them well so that you use the right word. A useful volume for this is The Merriam-Webster Pocket Dictionary of Synonyms. English dictionaries meant for learners also have a feature that brings out the exact differences between groups of synonyms. Longman Dictionary of Contemporary English is a good example. Two other sources are The Dictionary of Confusable Words by Laurence Urdang and Room’s Dictionary of Distinguishables. Several websites are devoted to explaining such differences; wiseGEEK and DifferenceBetween.net are good examples.
Lastly, prepositions. In English, one verb can mean many different things depending on the preposition that comes after the verb; to swear at, for instance, means something quite different from to swear by. To swear at somebody is to curse or to verbally abuse somebody, whereas to swear by something is to believe in it strongly. To understand such differences, dictionaries of phrasal verbs are particularly useful, for example, Cambridge International Dictionary of Phrasal Verbs and F T Wood’s English Prepositional Idioms. One of the many online resources you might find helpful is: