Questions & Answers about את בטוחה שזה המספר שלו?
Because the speaker is talking to one female person.
- את = you when speaking to one woman
- בטוחה = sure/certain in the feminine singular form
Hebrew usually makes adjectives agree with the person they describe.
If you were speaking to one man, you would say:
אתה בטוח שזה המספר שלו?
If you were speaking to a group, the forms would change again.
In Hebrew, the verb to be is usually not written in the present tense.
So English Are you sure...? becomes literally something like:
You sure that this is his number?
That is completely normal in Hebrew.
The sentence does not need a present-tense form of to be.