The word ‘which’ is a relative pronoun and is used to refer back to something that has already been mentioned. It can be used in both a singular and plural form, depending on the context of the sentence.

When using ‘which’ in a sentence, it typically follows an independent clause or phrase which provides additional information about what was previously stated, for example: “I bought some new shoes today, which were very comfortable.” In this instance, ‘which’ refers back to the shoes that were just mentioned to provide more detail about them being comfortable.

Another use for ‘which’ involves introducing non-essential clauses into sentences; these are phrases or ideas that could be removed from the overall structure without changing their meaning too much. An example would be: “My brother’s car broke down last week, which was annoying.” Here we see how adding this extra information does not change the primary point made by saying my brother’s car had broken down but instead adds further insight into how inconvenient it may have been at such an unexpected time…

Additionally, when using ‘which,’ you should always ensure there is no ambiguity over what exactly you are referring back to – if necessary, include other words within your sentence. Hence, so as not to confuse readers with multiple possible interpretations. This will make sure your writing remains clear and concise throughout.

All-in-all, understanding when best to use ‘Which’ can add clarity and depth of meaning within any written piece – making sure all points remain relevant yet succinctly expressed.