Sentence #1 has a demonstrative adjective. Sentence #2 has both a demonstrative adjective and a demonstrative pronoun. Sentence #3 uses a regular possessive pronoun. Sentence #4 is the only sentence that uses a present progressive tense.
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Study questions addressed to articles
1.Yo estaba
2.Tu estabas
3.Las niñas estaban
4.mis amigos y yo estábamos