Learn Dutch: Aangezien de e-mail uw project betreft, moet u deze ondertekenen. - Since the email concerns your project, you must sign it.

The word aangezien (since/as) is a subordinating conjunction. This means it introduces a dependent clause, which forces the conjugated verb (betreft) to move to the very end of that clause.

Why does the second half of the sentence start with the verb (moet u) instead of the subject (u moet)?

In Dutch, the conjugated verb must be the second idea in a main sentence. The entire first clause (Aangezien de e-mail uw project betreft) acts as the first block of information. Therefore, the main clause must start immediately with the verb (moet) to keep it in the second position, which pushes the subject (u) right after it.

Why is deze used to mean "it" here instead of hem or het?

Because e-mail is a de-word, you cannot use het. You could definitely use hem, but in formal written Dutch, it is very common to use demonstrative pronouns like deze (this one) or die (that one) to refer back to things. It sounds a bit more professional.

Is ondertekenen (to sign) a separable verb?