Learn English – Is using “I don’t know that” instead of “I don’t know if” grammatically correct

Because I'm older than she, I should be the first chosen.

I loved her and then she died.

If it rains, then it pours.

I've had more than enough.

Would you rather be a mouse than a rat?

Who, other than you, likes the color red?

When there is a comparison, you use than; then means:

1. at that time; at the time in question: I was living in Cairo then | [after preposition] Phoebe by then was exhausted | [as adjective] a hotel where the then prime minister, Margaret Thatcher, was staying.
2. after that; next; afterward: she won the first and then the second game.
   • also; in addition: I'm paid a generous salary, and then there's the money I've made at the races.
3. in that case; therefore: if you do what I tell you, then there's nothing to worry about | well, that's okay then.
   • used at the end of a sentence to emphasize an inference being drawn: so you're still here, then.
   • used to finish off a conversation: see you in an hour, then.

(See also the definition of then given in the Oxford Living Dictionaries.)

What I was taught in school was that it's acceptable to omit the comma when joining very short independent clauses. What's "short enough" is a judgment call.

I will happily agree with anyone that (a) it doesn't hurt to put the comma in, and (b) whenever the comma definitely makes things clearer, by all means it should be used.

In your particular case, my judgment is that your second independent clause is short enough that you could get away with omitting the comma. (I would include it, though.)