Learn French: Dans la rue, il faut être prudent. - In the street, you must be careful.

Il faut is an "impersonal" expression. This means the il doesn't mean "he" or any specific person. It literally means "it is necessary." English speakers often use "you" for general rules, but French uses il faut to say "one must" or "everyone must."

Why is the verb être (to be) left in its full infinitive form instead of being conjugated?

The rule for il faut is that it is always followed by an infinitive verb (the unconjugated dictionary form). Since il faut already acts as the conjugated verb in this part of the sentence, être stays exactly as it is.

Prudent is masculine singular. Does it change to prudente if a woman is reading or speaking this sentence?

Not in this exact sentence! Because il faut is a general statement applying to everyone, there is no specific person to match the adjective to. When making a general statement like this, French always defaults to the masculine singular form (prudent).

Why does it say dans la rue (in the street) instead of using a word for "on"?

In English, we often say "on the street," but in French, a street is thought of as a physical space bordered by buildings. Therefore, you are considered to be "in" it, so you use dans (in) rather than sur (on).