for, because, since, as

When they are used to give a reason, all of them have a very similar meaning but are used in different circumstances. Because is used when the reason is the most important part of the sentence or utterance. The 'because' clause usually comes at the end: "I went to Spain last summer because I wanted the guarantee of sunshine on every day of my holiday." If the speaker/writer wants to emphasise the importance of the reason, they may sometimes begin the sentence with the 'because' clause but it would be much commoner to see this in writing rather than in speaking because it indicates that the writer has (a) anticipated the question (see below) and (b) is making a specially important point about the reason for the choice of holiday: "Because I wanted guaranteed sunshine on every day of my holiday, I went to Spain last summer." If the question "Why did you go to Spain last summer?" had been asked, the reply would be simply "Because I wanted guaranteed sunshine on every day of my holiday." As and since are used when the reason is something already accepted or known and is therefore usually less important as a reason for the decision. The 'as' or 'since' clause is usually placed at the beginning of the sentence: "As the performance had already started, we went up to the balcony and occupied some empty seats there." "Since John had already eaten, I made do with a sandwich." For is used when the reason is given as an afterthought. It is never placed at the beginning of the sentence and is more characteristic of written, rather than spoken English: "I decided to stop the work I was doing, for it was very late and I wanted to go to bed." (The same person when speaking would probably use 'because' or 'as' in place of 'for' here. Not only are some words used more often than others when speaking but rhythm and assonance are much more important and the /oz/ sound or the /az/ sound of 'because' or 'as' probably work better with the /oz/ sound of 'was'.)