A ‣ Reason

I went home early because I was feeling tired.

As/Since I was feeling tired, I went home early. (We don’t usually start a sentence with because.)

I was feeling tired, so I went home early. (This is very common in spoken English.)

The reason I went home early was that I was feeling tired.

We can also use because of with a different construction. Compare:
They go there because the weather is wonderful. (because + noun + verb)
They go there because of the wonderful weather. (because of + (adjective) + noun)

Due to means the same as because of, and is often used to explain the reason for a problem.

The plane was late due to bad weather. (Due to is often used after the verb be.)

B ‣ Purpose

A purpose is an intention or reason for doing something.

The purpose of the meeting is to plan next year’s timetable. [the reason for the meeting]

We often introduce a purpose using so (that) or (in order) to:
I bought this book so (that) I could improve my English.
They went home early (in order) to watch the match on television.
We moved house so (that) we could send our children to this school.
She went into town (in order) to do some shopping.

C ‣ Result

These link words/phrases are used when one thing happens because another thing has happened. Therefore and as a result are more formal than so, and less common in spoken English.

I left my ticket at home, so I had to buy another one.

They’ve got more money, and therefore they can afford to buy the best football players.

I forgot to send the email. As a result, no one knew about the meeting.

D ‣ Conditions

We sometimes use whether [if] when we are not sure about something.
I didn’t buy it because I wasn’t sure whether you’d like it.
I spoke to the others, but I don’t know whether they’re coming.

We’ll be late unless we leave now. [if we don’t leave now]
Unless the weather improves [if the weather doesn’t improve], we won’t be able to go.

I have to go now, otherwise [because if I don’t] I’ll miss the last bus.
You’ll have to turn up the music, otherwise [because if you don’t] they won’t be able to hear it.

You can borrow it as long as you bring it back by Thursday. [but you must bring it back]
You can wear what you like as long as you look quite smart. [but you must look quite smart]

I’m taking my umbrella in case it rains. [I’m doing A because B might happen later.]
Take some money in case you need to get a taxi.