potere / dovere / volere

In Italian, these are called verbi servili. Here is how they function:

• Potere: Expresses physical ability, situational possibility, or asking for permission.
  Posso entrare? (May I come in?) Note: To express managing to do something despite an obstacle, use riuscire a (e.g., Non riesco a vedere).
• Dovere: Expresses a duty, a strong necessity, or a logical probability.
  Devo andare a casa. (I must go home.) Note: When used alone with a noun, it means 'to owe' (e.g., Ti devo dieci euro).
• Volere: Expresses a wish or a firm intention.
  Voglio comprare un gelato. (I want to buy an ice cream.)
• Sapere: When followed by an infinitive, it means 'to know how to' (a learned skill).
  So nuotare. (I know how to swim.)

Key Rules:
1. No Prepositions: Never say 'voglio di andare'. The infinitive follows immediately.
2. Irregularity: These verbs change significantly in the present tense (e.g., io posso, io devo, io voglio, io so).
3. Pronoun Placement: You can put pronouns before the modal verb (Lo voglio fare) or attach them to the end of the infinitive. When attaching a pronoun, you must drop the final '-e' of the infinitive (e.g., fare -> far- + lo = Voglio farlo).
4. Politeness: Use the conditional vorrei (I would like) instead of voglio to be more polite.
5. Compound Tenses: In tenses like the passato prossimo, the modal verb usually takes the auxiliary (essere or avere) required by the infinitive that follows it. For example, andare takes essere (Sono dovuto andare), while mangiare takes avere (Ho dovuto mangiare). However, with intransitive verbs, using avere is also correct (e.g., both è dovuto uscire and ha dovuto uscire are accepted). Note: Unlike the other modal verbs, sapere almost always takes the auxiliary avere in compound tenses, regardless of the infinitive that follows it (e.g., Ho saputo reagire), except when a clitic pronoun that triggers essere is placed before it (e.g., Ci sono saputo andare). If the infinitive is essere, the modal always takes avere (Ha dovuto essere forte). If the modal is used alone without an infinitive, it always takes avere.
6. Reflexive Pronouns in the Past: For reflexive and pronominal verbs, pronoun placement dictates the auxiliary verb in compound tenses. If the reflexive pronoun is placed before the modal, use essere (Mi sono dovuto svegliare). If it is attached to the infinitive, use avere (Ho dovuto svegliarmi).