A conjunction is a word used to join two words or clauses together. Note that a conjunction merely joins one word or clause with another word or clause. It does no other function in a sentence.

When

The conjunction when shows time. It can go in the middle of a sentence. Or it can go at the beginning.

The children went to the zoo. They saw several wild animals.
When the children went to the zoo, they saw several wild animals. OR The children saw several wild animals when they went to the zoo. (Note the use of comma in the first sentence.)

We can go to the park. The weather clears.
We can go to the park when the weather clears. OR When the weather clears, we can go to the park.)

The game finished. Everybody went home.
When the game finished, everybody went home. OR Everybody went home when the game finished.

And and But

Use and to join similar ideas. Use but to join contrasting ideas.

Alice took out her pen. She began to write.
Alice took out her pen and began to write. (Note that we leave out ‘she’ because both clauses have the same subject.)

Alice wrote a letter. Peter posted the letter.
Alice wrote a letter and Peter posted it. (Note the use of ‘it’ instead of ‘the letter’. We use pronouns to avoid the repetition of nouns.)

He was angry. He didn’t say anything. (Here the sentences express contrasting ideas. We normally expect angry people to shout, don’t we?)
He was angry but he didn’t say anything.

She is poor. She is honest.
She is poor but she is honest.

It was a sunny day. The wind was cold.

It was a sunny day but the wind is cold.

Because

Because shows reason.

He failed in the examination. His father was angry.
His father was angry because he failed in the examination.

She didn’t want to work. She was ill.
She didn’t want to work because she was ill.

She was happy. She got a prize.
She was happy because she got a prize.

After

After shows time. After can go either in the middle of a sentence or at the beginning.

I will play. I study my lessons.
I will play after I study my lessons. OR After I study my lessons, I will play. (Note the use of comma in the second sentence.)

We can go to the market. The rain stops.
We can go to the market after the rain stops. OR After the rain stops, we can go to the market.