Learn Portuguese (Portugal): Assim que ela estiver na cidade, integra-se. - As soon as she is in the city, she integrates herself.

The phrase assim que means "as soon as". When it introduces an action that hasn't happened yet, Portuguese requires the Future Subjunctive (Futuro do Conjuntivo). Even though English just uses the present tense "is", Portuguese uses estiver to show we are anticipating a future event.

How do we get the irregular form estiver for this tense?

For core irregular verbs, the Future Subjunctive is built from the simple past (Pretérito Perfeito). If you take the "eles" past form of estar, which is estiveram, and drop the -am, you get the base estiver. For ela, you don't add any extra ending, leaving it as estiver.

Why do we use the verb estar (estiver) here rather than ser (for)?

Both verbs translate to "to be", but Portuguese uses estar for physical locations. Since she is going to be "in the city" (na cidade), you must use estar, regardless of how long she plans to stay there.

Why is integra-se in the present tense if her integration will happen in the future?

In Portuguese, it is extremely common to use the present tense to talk about a guaranteed future result or an immediate consequence. Using the present tense integra-se instead of the future tense makes her integration sound like an absolute certainty once she arrives.