I thought this would be "אני בתוך את המסדעה שלהם", don't you need the "את" as I (as a subject) am speaking about an object?
It's not the object of a verb so no.
Am I right in thinking that this is because "בתוך" is a preposition and not a verb?
the word בתוך serves as the preposition "in" (indirect object/dative), so you cannot use את here, which is the preposition for the direct object/accusative.
Why not בתוכה?
Because בתוכה is like "inside her" (or "inside it", which would be the case for "restaurant", translating to English). You wouldn't say "I am inside it the restaurant". Or the subject, or the pronoun, not both.
Why isn't is "אני בתןכי
Could I say "אני בתוך מסעדהם"?
oops....I didn't answer it correctly...silly
Why is this sentence given wrong? Its not clear if its m or f
I’m assuming you’re talking about the last word “their restaurant”. The English is not clear whether it’s masculine or feminine, so in this case Duolingo usually accepts either.
The version which Duolingo chose was masculine, which is clear from shelahem.