Answer:
1) juegan, 2) pierde, 3) empiezan, 4) encuentra, 5) vuelven.
Explanation:
1) Los niños <em>juegan</em> en el parque. (The sentence is written in present tense. The subject <em>Los niños</em> belongs to the third person plural).
2) Josué <em>pierde</em> la pelota de Marcos. (The sentence is written in present tense. The subject <em>Josué</em> belongs to the third person singular).
3) Marcos y Josué <em>empiezan</em> a buscar la pelota. (The sentence is written in present tense. The subjects <em>Marcos y Josué</em> belong to the third person singular).
4) Mónica <em>encuentra</em> la pelota cerca del monumento. (The sentence is written in present tense. The subject <em>Mónica</em> belongs to the third person singular).
5) Marcos, Josué y Mónica <em>vuelven</em> a jugar. (The sentence is written in present tense. The subjects <em>Marcos, Josué y Mónica</em> belong to third person plural).