Because introduces clauses of cause and reason. It is a subordinating conjunction. This means that the clause it introduces is a subordinate clause, which needs a main clause to make it complete. We use a comma when the subordinate clause comes before the main clause:
[main clause]Everyone left earlybecause[subordinate clause]Mark and Helen had an argument.
[subordinate clause]Because they were so tired, [main clause]they went to bed at 9 pm.
We don’t use for or why instead of because when we are giving reasons:
I’m going to go to the company’s head office on Monday because there is an emergency meeting there.
Not: … to the company’s head office on Monday for/why there is an emergency …
Because of is a two-word preposition meaning ‘as a result of’:
Because of the rain, the tennis match was stopped.
There were so many people in the shop because of the sale.
Cos, a short form of because, is pronounced /kəz/ or /kɒz/ and can also be spelt ’cause. It can be used instead of because (and cos of instead of because of). We often use it in speaking, emails and text messages, especially in informal situations:
Why are you and Adam not talking to each other?
Coshe’s always saying nasty things about me.
I don’t eat meat cos I love animals.
They cancelled the trip cosof the snow.
Just because, simply because
We can emphasise because with just or simply:
Just because you’re the boss, it doesn’t mean you can be rude to everyone.
I don’t want to do it simply because I think it’s wrong.