Learn French: Tu devrais t'appliquer, ton attitude est mauvaise. - You should apply yourself, your attitude is bad.

You would use Je veux que followed by the subjunctive form of the verb: Je veux que tu t'appliques. Our current sentence uses tu devrais (you should), which is a statement of advice using the conditional mood, so no subjunctive is triggered here.

Why use devrais instead of dois?

Devrais is the conditional form of devoir (to have to / must). Using the conditional turns the strict obligation of "you must" (tu dois) into the softer advice of "you should" (tu devrais). This is the standard way to give advice in French.

Why is it t'appliquer and not just appliquer?

The verb s'appliquer is reflexive when it means "to apply oneself" or "to put effort into something". If you used the non-reflexive appliquer, it would mean to physically apply one thing to another (like applying cream to your skin).

Attitude is a feminine noun. Why does the sentence use the masculine possessive ton instead of ta?

Even though attitude is feminine, it begins with a vowel. French requires you to use the masculine possessive adjectives (mon, ton, son) before any singular feminine noun starting with a vowel or a mute H. This avoids the awkward pause that would happen if you tried to say ta attitude.