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Answer:

To solve the conjugation of stem - changing verbs (e - ie) and write sentences with them, we follow these steps:

Conjugation of "Preferir" (e - ie stem - changing verb)

Step 1: Yo form

For the first - person singular (yo), we change the stem "e" to "ie" in the present tense. The verb "preferir" has the ending - o for yo. So, it is $\boldsymbol{\text{prefiero}}$.

Step 2: Tú form

For the second - person singular (tú), we change the stem "e" to "ie" and use the ending - s. So, it is $\boldsymbol{\text{prefieres}}$.

Step 3: Él/Ella/Usted form

For the third - person singular (él/ella/usted), we change the stem "e" to "ie" and use the ending - e. So, it is $\boldsymbol{\text{prefiere}}$.

Step 4: Nosotros form

For the first - person plural (nosotros), we do not change the stem (we use the regular "e") and use the ending - imos. So, it is $\boldsymbol{\text{preferimos}}$.

Step 5: Vosotros form

For the second - person plural (vosotros), we do not change the stem and use the ending - ís. So, it is $\boldsymbol{\text{preferís}}$.

Step 6: Ellos/Ellas/Ustedes form

For the third - person plural (ellos/ellas/ustedes), we change the stem "e" to "ie" and use the ending - en. So, it is $\boldsymbol{\text{prefieren}}$.

Conjugation of "Pensar" (e - ie stem - changing verb)

Step 1: Yo form

For the first - person singular (yo), we change the stem "e" to "ie" and use the ending - o. So, it is $\boldsymbol{\text{penso}}$ (wait, no, correction: "pensar" is a stem - changing verb where "e" changes to "ie" in the present tense for all forms except nosotros and vosotros. So for yo: $\boldsymbol{\text{pienso}}$ (I think I made a mistake earlier, let's redo. The correct conjugation: For "pensar", the stem is "pens - ", and we change "e" to "ie" in the present tense for non - nosotros/vosotros forms. So yo: pienso, tú: piensas, él/ella/usted: piensa, nosotros: pensamos, vosotros: pensáis, ellos/ellas/ustedes: piensan. Let's correct the steps properly.

Step 1: Yo form (correct)

For "pensar", yo form: change "e" in "pens - " to "ie", add - o. So $\boldsymbol{\text{pienso}}$.

Step 2: Tú form (correct)

Change "e" to "ie", add - s. So $\boldsymbol{\text{piensas}}$.

Step 3: Él/Ella/Usted form (correct)

Change "e" to "ie", add - e. So $\boldsymbol{\text{piensa}}$.

Step 4: Nosotros form

We do not change the stem (keep "e"), add - amos. So $\boldsymbol{\text{pensamos}}$.

Step 5: Vosotros form

We do not change the stem, add - áis. So $\boldsymbol{\text{pensáis}}$.

Step 6: Ellos/Ellas/Ustedes form

Change "e" to "ie", add - an. So $\boldsymbol{\text{piensan}}$.

Conjugation of "Empezar" (e - ie stem - changing verb)

Step 1: Yo form

For the first - person singular (yo), we change the stem "e" to "ie" and use the ending - o. So, it is $\boldsymbol{\text{empiezo}}$.

Step 2: Tú form

For the second - person singular (tú), we change the stem "e" to "ie" and use the ending - s. So, it is $\boldsymbol{\text{empiezas}}$.

Step 3: Él/Ella/Usted form

For the third - person singular (él/ella/usted), we change the stem "e" to "ie" and use the ending - e. So, it is $\boldsymbol{\text{empieza}}$.

Step 4: Nosotros form

For the first - person plural (nosotros), we do not change the stem (keep "e") and use the ending - amos. So, it is $\boldsymbol{\text{empezamos}}$.

Step 5: Vosotros form

For the second - person plural (vosotros), we do not change the stem and use the ending - áis. So, it is $\boldsymbol{\text{empezáis}}$.

Step 6: Ellos/Ellas/Ustedes form

For the third - person plural (ellos/ellas/ustedes), we change the stem "e" to "ie" and use the ending - an. So, it is $\boldsymbol{\text{empiezan}}$.

Writing sentences with each verb

Sentences with "Preferir"

1. Yo prefiero el chocolate sobre la vainilla. (I prefer chocolate over vanilla.)
2. Tú prefieres estudiar en la biblioteca, ¿verdad? (You prefer to study in the library, right?)
3. Ellos prefieren jugar fútbol los sábados. (They prefer to play soccer on Saturdays.)

Sentences with "Pensar"

1. Yo pienso que mañana va a llover. (I think that tomorrow it is going to rain.)
2. Tú piensas demasiado sobre eso. (You think too much about that.)
3. Ella piensa que la clase es interesante. (She thinks that the class is interesting.)

Sentences with "Empezar"

1. Yo empiezo a trabajar a las ocho. (I begin to work at eight.)
2. Tú empiezas el proyecto hoy. (You begin the project today.)
3. Ellos empiezan la clase con un saludo. (They begin the class with a greeting.)

Final Conjugation Tables

Table for "Preferir"

Subject	Conjugation
Tú	prefieres
Él/Ella/Usted	prefiere
Nosotros	preferimos
Vosotros	preferís
Ellos/Ellas/Ustedes	prefieren

Table for "Pensar"

Subject	Conjugation
Tú	piensas
Él/Ella/Usted	piensa
Nosotros	pensamos
Vosotros	pensáis
Ellos/Ellas/Ustedes	piensan

Table for "Empezar"

Subject	Conjugation
Tú	empiezas
Él/Ella/Usted	empieza
Nosotros	empezamos
Vosotros	empezáis
Ellos/Ellas/Ustedes	empiezan

If you only wanted the conjugation part, the conjugations are as shown in the tables. If you wanted the sentences, the sentences are also provided.