To be

The verb "to be" in Swahili is very simple, since it isn't actually a verb at all (in grammatical terms, it's called a copula).  The only word you need is ni:

Mimi ni daktari          I am a doctor
Yeye ni Mtanzania     He/She is Tanzanian
Wao ni wanafunzi      They are students

The negative form (am not, is not) is equally simple: si.

Wewe si mwalimu     You are not a teacher
Sisi si Waingereza      We are not English
Mimi si pweza            I am not an octopus

Because these don't change form, you need to use personal pronouns (I, you, he, she etc.) to specify who or what you're talking about.  These are:

Mimi      I
Wewe    You
Yeye      He / She
Sisi        We
Nyinyi   You (plural)  [sometimes spelt 'ninyi']
Wao       They   [sounds like 'wow!']

Asking questions is easier than in English, because you don't even have to change the word order.

Wewe ni mwalimu?    Ndiyo, mimi ni mwalimu.
Are you a teacher?   Yes, I'm a teacher.

Nyinyi ni Wakenya?  Hapana, sisi si Wakenya. Sisi ni Wazanzibari.
Are you Kenyan?    No, we're not Kenyan.  We're Zanzibari.

Often the word je is used to alert the listener to the fact that a question is coming, but the meaning remains the same:

Je, yeye ni fundi*?          Is he a fundi?
Je, nyinyi ni wapishi?    Are you cooks?

* It's hard to translate the word 'fundi' into English.  A fundi could be a mechanic, a bicycle repairman, an electrician, a carpenter, a tailor, a plumber, or anyone with some kind of useful manual skill - usually either a craftsman or someone who can fix things.