I went to the park, it was very nice

In English, a comma cannot connect clauses.  Instead:
1) use two sentences or
2) use a conjunction or
3) use a subordinate clause or
4) use stronger punctuation

1) Use two sentences

The easiest way to avoid the weak comma problem is to make two sentences.

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

2) Use a conjunction

Conjunctions tie parts of sentences together,
Conjunctions include: and, but, so, or.

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

3) Use a subordinate clause

Subordinate clauses are clauses used to describe another clause or object.
Subordinate clauses are put next to the thing they are describing.
Subordinate clauses usually begin with which, that, or who.

Wrong	Right
I went to the park, it was very nice.	I went to the park, which was very nice.
My teacher was very angry, he told me to sit down.	My teacher was very angry, which made him tell me to sit down.

4) Use stronger punctuation

One way to avoid the weak comma problem is to replace the comma with stronger punctuation.
Usually the new punctuation is a colon or semi-colon.
Strong punctuation can look unusual in short sentences. Therefore only use colons or semi-colons occasionally.

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

Exercise

Re-write these sentences to make them correct:

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.