made of the dictionary form of a verb plus koto ga aru 辞書形+ことがある
-ta form + koto ga aru/arimasu means "ever" or "once" -ta form + koto ga nai/arimasen means "never" or "not once"
Affirmative: I once studied Spanish. スペイン語を勉強したことがある。
Negative: I've never studied Spanish. スペイン語を勉強したことがない。
Tigertさん: Thank you for translating "a little better" and for "making some changes". I translated #6548 because Chohongsieudieuさん didn't. I am not a Japanese native speaker, neither an English native speaker. This is the reason why I all always put my contributions as suspicious. When you think they are OK, you may turn them to neutral.
wouldn`t this translate a little better as `there are times when`, sometimes already has a word 時々 and since the grammar point uses the existance verb ある, including it in the translation makes it a little easier to remember. I hope you don`t mind but I made some changes to the above examples. nosebleed is a noun so it requires a preposition article "solution way" is redundant because a solution is a way 相談する means to consult, 会話する means to conversate. 研究室 translates to research room. of course it can also mean the professor`s office, but one would never say "at your professor`s office"