So how do I know which article to use in English? Would "the gast" be expressed differently?
is it normal to not use pronouns in swahili? so, a native would usually or always say "ni mgeni"?
Ni mgeni doesn't say who is a guest. It could be you, he, she, it, we, they depending on context. If the subject is shown on the verb, you usually drop the pronoun. If it's not shown on the verb (such as with ni, si and hu-, which are invariable) the pronoun is used.