I'm a native English speaker and I'd say: "Which one would you choose?" —Using the conditional tense of the verb "to choose" which is "would choose". "What would you say when there is something happy happening?" / "What would you say when something happy is happening?" — Again, using the condition tense of "to choose". — "Are" should be conjugated as "is" because "something" is a third-person singular pronoun. — "Happen" should be "happening" because the verbal phrase is "to be happening". ("Something is happening") Sorry about the complicated grammar :-)

thank you for your help

Vitor is right about the first one. The second one should be "What you will say when * something happy happens?"

xxx

thank you for your help. let's wait together.