Learn Italian: Sai parlare una lingua straniera? - Do you know how to speak a foreign language?

Sapere is used when you are talking about an acquired skill or knowing how to do something. Conoscere means to be familiar with a person, place, or thing, and it is never followed by an action verb like parlare (to speak).

Why is there no word for "how to" between sai and parlare?

In Italian, to say you "know how to" do something, you simply use the correct form of sapere directly followed by the infinitive (the unchanged dictionary form) of the action verb. You don't need any extra connecting words.

Could I use conoscere to ask someone if they know a foreign language?

Yes, but you would have to drop the verb parlare. You could ask Conosci una lingua straniera? (Are you familiar with a foreign language?). As soon as you include the action of speaking (parlare), you are talking about a skill, so you must switch back to sapere.

Why does the word for foreign (straniera) end in -a?

Adjectives in Italian must match the gender and number of the noun they describe. The word for language (lingua) is feminine and singular, so the base adjective straniero changes its ending to -a to match.