Reason, purpose and result

Reason

I went home early because/as/since I was feeling a bit tired.
Note: With as or since, the reason (in this example ‘feeling tired’) is often known to the listener or reader, so it is less important. It is also common to put as/since at the beginning of the sentence: ‘As/since I was feeling tired, I went home early’. In spoken English, many native speakers would use so after the reason: I was feeling a bit tired, so I went home early, We can also use because of, but with a different construction. Compare:
We always go there because the weather is absolutely wonderful, (because + noun + verb) We always go there because of the wonderful weather, (because of + (adjective) + noun)
Due to and owing to have the same meaning as because of, but they are more formal, and are often used in sentences which explain the reason for a problem:
The plane was late due to bad weather, (due to is often used after the verb ‘to be’)
Due to / Owing to the power cut last night, I missed the late film on TV.

‘Cause’ and ‘result’ verbs

There are some verbs which we can use in similar ways to the words above:
Police think the bus caused the accident. (= was responsible for the accident)
The extra investment should lead to more jobs. (= result in more jobs)
‘Cause’ and ‘result’ verbs sometimes appear together in this way:
Police think that a cigarette caused the fire which resulted in the destruction of the building.

Purpose

A ‘purpose’ is an intention, an aim or a reason for doing something:
The purpose of buying this book was to improve my English.
But we often introduce a purpose using so (that):
I bought this book so (that) I can improve my English.
They went home early so (that) they could watch the match on television.
We moved house so (that) we could send our children to this school.
Note: In spoken English, people often just say so (without that). It is also very common (as in the examples) to use a modal verb, e.g. can or could, after so that.

Result

These words introduce a result:
I left the ticket at home, so I’m afraid I had to buy another one.
I forgot to send the letters. Consequently, some people didn’t know about the meeting.
She was extremely hard-working and therefore deserved the promotion.
Both the manager and his assistant were ill. As a result, there was no-one to take decisions.
Note: So is the most common, and usually links ideas in a single sentence. As a result and consequently are more formal, and usually connect ideas in two separate sentences (as in the examples). Therefore (also more formal), can be used in a single sentence (as in the example), but may also connect two sentences.