Learn Portuguese (Portugal): Olá, como te chamas? - Hello, what is your informal name?

While European Portuguese normally attaches the pronoun to the end of the verb (a rule called enclisis), certain words act like magnets and pull the pronoun to the front. Question words like como (how) are magnet words! Because of como, the pronoun te jumps before the verb chamas.

Why do we use como (how) to ask for a name instead of the word for 'what'?

In English, you ask 'what is your name?', but Portuguese uses the reflexive verb chamar-se, which literally means 'to call oneself'. So, como te chamas? translates literally to 'how do you call yourself?'.

Where is the word for 'you' in this sentence?

It is hidden in the verb! In Portuguese, you often drop subject pronouns like tu (informal you) because the verb ending tells you who is acting. The -s at the end of chamas, plus the pronoun te, already make it perfectly clear that you mean 'you'.

How would I ask this same question politely to a stranger?

This sentence uses te and chamas, which belong to the informal tu (you). If you are speaking to a stranger, an elder, or in a formal setting, you should use the formal 'you' forms. The polite version of this question is Como se chama?