What type of conditional is this? "If Julie went to the party last night, she definitely saw what happened."

Solution 1:

The conditionals taught in EFL lessons are taught as examples of how conditionals can be. They aren't meant to be models of every type of conditional. They are meant to be patterns to practice when you are starting to learn the language. They shouldn't be understood as an exhaustive description of every type of tense and modal verb possible in conditional sentences.

There are really only two types of conditionals, those that use tense in the same way as other sentences and those that use backshifted verb forms ( examples of which would be those conditionals termed second or third conditionals in language teaching).

In the Original Poster's example, the tenses are being used in the normal way. In other words, the past simple verb forms are just being used to indicate past time.

The sentence just indicates that the speaker guarantees the truth of Julie saw what happened if the proposition Julie went to the party is true.

Solution 2:

I taught at an American English institute that categorized conditionals based on "real" (zero/first) or "unreal" (second/third). In either case conditional sentences can be "mixed" rather than following the standard setup.

The examples listed are examples of mixed real conditionals.

"If Julie went to the party last night, she definitely saw what happened."

This is referring to a real event rather than a hypothetical one and refers to two past actions. Both clauses can be changed to represent different times.

ie: If Julie went to the party last night, she probably won't go out tonight.

This is based on a real situation and a prediction about the future.

The "unreal" conditionals can be mixed as well.

ie: If you had won $100 yesterday at the casino, would you go out to dinner with us tonight?

This is based on an unreal event (you didn't win the money yesterday) and offering a hypothetical situation about the future (going out to dinner tonight).