Learn Italian: Credo che Lei debba leggere il riepilogo, altrimenti perderà la riunione. - I believe that you must read the summary, otherwise you will miss the meeting.

Altrimenti means "otherwise" or "if not". It acts as a conditional connector linking a requirement (reading the summary) to the negative consequence of failing to meet it. Because the consequence will happen in the future, it is logically followed here by the future tense verb perderà.

Why is the verb dovere conjugated as debba here?

Debba is the present subjunctive form of dovere (must). It is required here because the sentence starts with Credo che (I believe that), an expression of opinion that triggers the subjunctive mood for the verb in the dependent clause.

How do we know who debba refers to, since the subjunctive forms for I, you, he, and she are identical?

The present subjunctive of dovere is debba for all singular persons. The speaker includes the capitalized pronoun Lei right before the verb to clarify that they are addressing the listener formally. Without Lei, it could ambiguously mean "I believe that I must read..." or "I believe that he must read...".

Why is the verb perdere used for "miss" in perderà la riunione?