Modal verbs express possibility, ability, obligation, permission, or intention. They work together with a verb in the infinitive to complete the meaning. In Spanish, the main modals are poder, deber, querer, and saber. They are used to nuance the meaning of a sentence without changing the main verb. They typically appear before the infinitive in the sentence.

Can expresses possibility or permission depending on the context. It can also indicate ability in concrete situations. It combines with an infinitive to form the full sense. In the present, its root is irregular for some persons. In the past tense, it changes meaning to 'managed' or 'succeeded' in many contexts.

Should expresses obligation or a strong recommendation. It can be softened with expressions like 'should' to sound more polite or less categorical. It is used with the infinitive to indicate what is necessary or right to do. In the imperfect past, it often indicates an obligation not fulfilled or pending.

Want expresses intention or desire. With an infinitive, it indicates what a person plans or wants to do. In some contexts it can sound direct, and 'would like' is preferred for more courtesy. In the past tense, it usually means an attempt and can change nuance if used in the negative.

Know, as a modal, expresses learned ability or practical knowledge. It is used with the infinitive to say that someone is capable of doing something because they have learned it. It differs from poder, which usually refers to possibility or permission in a situation. It can also contrast with conocer, which is used for people or places.

The meaning of a modal can change depending on context and the speaker's intention. Can varies between possibility and permission depending on who sets the condition. Should may sound stronger or weaker depending on the tone and words that accompany it. Want can be literal or function as a request, especially with more courteous forms. Know with the infinitive focuses on learned ability, not momentary opportunity.

Modal verbs are placed directly before the infinitive without an intermediate preposition. Negation usually goes before the modal to negate possibility, obligation, intention, or ability. In questions, the modal remains in front of the infinitive. In longer structures, the modal remains the nucleus that determines the main nuance.

Can, should, want, and know are used with the infinitive to express possibility, obligation, intention, and ability. Their present conjugation is key to constructing clear sentences. Context decides whether the modal is interpreted as permission, desire, or ability. Choosing the correct modal allows communicating precise nuances in Spanish.