Learn Dutch: Wacht een moment, alsjeblieft.

This sentence is a command or request, which is called the imperative form. In Dutch, you form a command by simply using the verb stem without a subject. Since the full verb is wachten, the stem is wacht.

Why is alsjeblieft used here instead of alstublieft?

alsjeblieft is the informal word for please, used with friends, family, or people you address as jij. If you were speaking to a stranger, an older person, or someone you address formally with u, you would say Wacht een moment, alstublieft.

Could I use even instead of een moment?

Yes, absolutely! Wacht even, alsjeblieft is extremely common in spoken Dutch and means exactly the same thing. een moment is correct but sounds slightly more literal.