• I ate a sandwich, and I drank a coke. (compound)
• I ate a sandwich and drank a coke. (hybrid)

In the first sentence (the compound sentence), the subject ("I") is repeated twice, once in the first clause and once in the second clause. Furthermore, there is a comma before "and".

In the second sentence (the hybrid sentence), there is no comma after "and", and the subject of the second clause is omitted (left out). In other words, who drank the coke? I did. But you don't have to say "I drank the coke", because the reader understands that the person who drank the coke is the same person who ate a sandwich.

The structure of the first sentence (the compound sentence) is

• [subject] did this, and [subject] did that.

It's just a coincidence that the subject of both clauses is the same.

For the second sentence (the hybrid sentence), the structure is:

• [subject] did this and did that.

It's one person doing two different things.

Hope this helps!