When to use Which?

Many ESL speakers use what and which interchangeably when asking questions, but they are not interchangeable.

The problem is not what, but which.

Which is used when:
1) asking for information about one of a limited number of things
"Which car is yours?"
"Which Tupperware is yours?"

2) asking about choice
"Which meal do you want?"

"Which is worse?"

For example, you would not say:

"Which was the name of your previous company?" (incorrect) because there are millions of companies

"Which name are you going to give the baby?" (incorrect) because there are countless names.

BUT, if you know the parents are choosing from a short list of names, you could then ask "Which name are you leaning towards?" (correct)