"What is Jack doing now?" is how you would say "杰克现在在干什么" in Engpsh.

"What" is a pronoun that is used to ask about the identity of the person or thing in question.
"Is" is the third person singular form of the verb "be" which is used to indicate the present tense.
"Jack" is a proper noun that refers to a specific person.
"Doing" is the present participle form of the verb "do" which is used to indicate that the person is engaged in some kind of activity or action.
"Now" is an adverb of time that indicates the present moment.

Examples:

What is Jack doing now?
Can you tell me what Jack is doing now?
I wonder what Jack is up to now.

中文翻译：

杰克现在在干什么？
你能告诉我杰克现在在干什么吗？
我想知道杰克现在在干什么。