Sometimes student write sentences such as these:

I went to the park, it was very nice.
My teacher was very angry, he told me to sit down.

However, in English these sentences are written incorrectly.
A comma cannot be used as a conjunction.

Solving the Problem

There are different ways to make the sentence correct.

a) Split the sentence into two sentences.

I went to the park. It was very nice.
My teacher was very angry. He told me to sit down.

b) Use a conjunction (and, but, so, because…)

I went to the park, and it was very nice.
My teacher was very angry, so he told me to sit down.

c) Create a ‘subordinate clause’ (that…, which…)

I went to the park, which was very nice.
My teacher was very angry, which was why he told me to sit down.

d) Use different punctuation ( : ; () – -)

I went to the park; it was very nice.
My teacher was very angry: he told me to sit down.

Exercise

Think of ways to re-write these sentences:

1. He smokes, it is very bad for him.
2. We will go to the zoo, I like the animals.
3. She broke up with her boyfriend, she doesn’t love him anymore.
4. They ate the cake, it was delicious.
5. The cat fell over, everybody laughed.