Welcome to your study guide on modal verbs and infinitive constructions! Learning how to use these verbs is one of the quickest ways to level up your Spanish. Instead of only being able to say simple things like "Como pizza" (I eat pizza), modal verbs allow you to express what you want to do, what you can do, what you must do, and what you know how to do.

Don't worry if verb conjugations have felt tricky in the past. Modal verbs follow one simple, golden formula that makes building long, impressive sentences much easier than you think!

1. The Golden Rule and Formula

A modal verb is a "helping verb" that expresses desire, possibility, or obligation. In Spanish, it is almost always followed immediately by a second verb that carries the main action.

The Golden Formula:
[Conjugated Modal Verb] + [Infinitive Verb]

What is an infinitive?
An infinitive is the base form of a verb before it has been changed (conjugated). In Spanish, all infinitives end in -ar, -er, or -ir (for example: hablar, comer, vivir). In English, this translates to "to speak", "to eat", or "to live".

The Golden Rule:
Only conjugate the first verb (the modal verb) to match the person doing the action. The second verb must always stay in its original infinitive form.

Examples:
• Quiero comer = I want to eat. (Quiero is conjugated; comer stays as an infinitive).
• Podemos bailar = We can dance. (Podemos is conjugated; bailar stays as an infinitive).
• Debes estudiar = You must study. (Debes is conjugated; estudiar stays as an infinitive).

Analogy to help you remember:
Think of the two verbs like a tandem bicycle. Only the person in the front seat steers and pedals (the conjugated modal verb). The person in the back seat just enjoys the ride in their normal clothes without doing any steering (the unconjugated infinitive verb)!

Key Takeaway: Never conjugate both verbs next to each other. Conjugate the first verb, and leave the second verb in its -ar, -er, or -ir infinitive ending.

2. The Five Key Modal Verbs

For KS3 Spanish, there are five essential modal verbs and constructions you need to know. Let's look at each one step-by-step.

1. Querer (To want)

Querer is a stem-changing verb. The letter e in the middle changes to ie for all people except nosotros (we) and vosotros (you all).

• yo quiero = I want
• tú quieres = you want
• él / ella quiere = he / she wants
• nosotros queremos = we want (no stem change)
• vosotros queréis = you all want (no stem change)
• ellos / ellas quieren = they want

Example: Ellos quieren salir. (They want to go out.)

2. Poder (To be able to / Can)

Poder is also a stem-changing verb. The letter o changes to ue in all forms except nosotros and vosotros.

• yo puedo = I can / I am able to
• tú puedes = you can / you are able to
• él / ella puede = he / she can / is able to
• nosotros podemos = we can / we are able to (no stem change)
• vosotros podéis = you all can / you all are able to (no stem change)
• ellos / ellas pueden = they can / they are able to

Example: ¿Puedes ayudar? (Can you help?)

3. Deber (Must / Should)

Deber is a standard, regular -er verb. It follows the regular present tense endings perfectly!

• yo debo = I must / I should
• tú debes = you must / you should
• él / ella debe = he / she must / should
• nosotros debemos = we must / we should
• vosotros debéis = you all must / you all should
• ellos / ellas deben = they must / they should

Example: Debemos escuchar. (We must listen.)

4. Saber (To know how to)

Saber is used with an infinitive to express knowing how to do an acquired skill. Notice that the yo form is irregular (sé with an accent mark), while the rest follow regular -er patterns.

• yo sé = I know how to (irregular)
• tú sabes = you know how to
• él / ella sabe = he / she knows how to
• nosotros sabemos = we know how to
• vosotros sabéis = you all know how to
• ellos / ellas saben = they know how to

Example: Sé hablar español. (I know how to speak Spanish.)

5. Tener que (To have to)

Tener que is a special structure (called a verbal periphrasis) used to show strong obligation. You conjugate tener and you must keep the word que before adding your infinitive.

• yo tengo que = I have to
• tú tienes que = you have to
• él / ella tiene que = he / she has to
• nosotros tenemos que = we have to
• vosotros tenéis que = you all have to
• ellos / ellas tienen que = they have to

Example: Tengo que hacer los deberes. (I have to do homework.)

Key Takeaway: Memorise the present tense conjugations of these five verbs. Once you conjugate the modal verb, simply place an infinitive verb straight after it!

3. Important Usage Rules

Making Sentences Negative

To make a sentence negative, place the word no directly before the conjugated modal verb.

Formula: No + [Conjugated Modal Verb] + [Infinitive Verb]

• No puedo bailar. = I cannot dance.
• No queremos ir. = We do not want to go.
• No tienes que pagar. = You do not have to pay.

Crucial Distinction: Saber vs. Poder

In English, we often use the word "can" for two different meanings: knowing how to do something and having permission or physical ability. In Spanish, these are two separate verbs:

• Saber + Infinitive: Used for learned skills (knowing how to do something).
Sé nadar. = I know how to swim (I learned the skill).
Ella sabe tocar el piano. = She knows how to play the piano.

• Poder + Infinitive: Used for physical ability or permission at a specific time.
Puedo nadar hoy. = I can swim today (I have permission / my arm isn't broken / the pool is open).
No puedo ir a la fiesta. = I cannot go to the party (I am not allowed / I am busy).

Key Takeaway: Put no before the first verb to make it negative. Use saber for skills you have learned and poder for whether you are able or allowed to do something.

4. Common Pitfalls to Avoid

Look out for these frequent mistakes in your tests and written work:

1. Double Conjugation (The Most Common Error)
• Incorrect: Quiero como pizza.
• Correct: Quiero comer pizza.
Remember: Only conjugate the first verb!

2. Forgetting the word "que" in "tener que"
• Incorrect: Tengo estudiar.
• Correct: Tengo que estudiar.
Without que, the structure does not work.

3. Stem-changing in the "nosotros" or "vosotros" forms
• Incorrect: Nosotros quiéremos ir. / Podémos jugar.
• Correct: Nosotros queremos ir. / Nosotros podemos jugar.
The nosotros and vosotros forms keep the regular vowel stems (e in querer, o in poder).

4. Confusing "Saber" and "Poder" for skills
• Incorrect: Puedo hablar francés (when meaning you learned the language).
• Correct: Sé hablar francés. (It is an acquired skill, so use saber).

5. Quick Summary Checklist

• Structure: Modal verb (conjugated) + Main verb (infinitive ending in -ar, -er, or -ir).
• Querer (e > ie): Expresses desire (quiero, quieres, quiere, queremos, queréis, quieren).
• Poder (o > ue): Expresses ability or permission (puedo, puedes, puede, podemos, podéis, pueden).
• Deber: Expresses duty or "must/should" (debo, debes, debe, debemos, debéis, deben).
• Saber: Expresses acquired knowledge/skill (sé, sabes, sabe, sabemos, sabéis, saben).
• Tener que: Expresses obligation (tengo que, tienes que, tiene que, tenemos que, tenéis que, tienen que).
• Negatives: Add no right in front of the modal verb (e.g. No debo comer chocolate).