The missing words or phrases that fill the blanks are:
<em>Martín trabaja en una escuela </em><u><em>pequeña</em></u><em>.</em>
<em>Los niños son </em><u><em>tontos</em></u><em>.</em>
<em>La esposa de David es </em><u><em>japonesa</em></u><em>.</em>
Explanation:
The translation of the sentences is:
Martín works in a <u>small</u> school.
The children are <u>dumb</u>.
David's wife is <u>Japanese</u>.
The adjectives in Spanish, unlike in English, <u>refer to the number and in most cases to gender</u>, and to which they refer depends on the noun, therefore, you must first identify the gender and the number that handles each one of the nouns used in the sentences:
<em>Escuela:</em> female and singular.
<em>Niños:</em> male and plural.
<em>Esposa:</em> female and singular.
Now you should review the form that each of the adjectives used can take depending on the gender and number:
Female and singular:<u><em>pequeña</em></u><em>, tonta, </em><u><em>japonesa</em></u><em>.</em>
Male and singular: <em>pequeño, tonto, japonés.</em>
Female and plural: <em>pequeñas, tontas, japonesas.</em>
Male and plural: <em>pequeños, </em><u><em>tontos</em></u><em>, japoneses.</em>
Finally, you should only identify the gender and number of the noun and use the adjective that has the same gender and number.