Learn French: Je veux que tu saches utiliser ce sifflet. - I want you to know how to use this whistle.

The phrase Je veux que expresses a desire or a wish. In French, verbs that express a wish, want, or demand always require the subjunctive mood in the following clause. That is why we use saches instead of the regular present tense sais.

How is the subjunctive form saches created?

The verb savoir is one of a few common verbs with a completely irregular subjunctive stem. Instead of taking the stem from the ils present tense form like most verbs do, it uses the special stem sach-. You just add the regular subjunctive endings to it, making it que tu saches.

The English translation says 'know how to use', but there is no French word for 'how' here. Why?

In French, when you want to say you know how to do a skill or action, you just use the verb savoir followed directly by an infinitive verb. You do not add comment for 'how'. So, savoir utiliser perfectly translates to 'to know how to use'.

Why do we use ce for 'this' instead of cet or cette?

The word sifflet (whistle) is a masculine noun that starts with a consonant, so it takes the masculine singular demonstrative adjective ce. You would only use cet before a masculine noun if it started with a vowel or a mute H (like cet ami), and cette is strictly for feminine nouns.