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У Анны был новый телефон.

У моих знакомых была вечеринка.

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VERB  <= NOUN №1
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When we need to decide about the form of a verb, we have to check the noun in Nominative case (form №1) regardless its meaning.

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Thus, in the first sentence the noun №1 is “телефон”, он (masculine). Therefore we use the verb in masculine: “был”, even though the Subject is feminine (Анна).

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In the second sentence the noun №1 is “вечеринка”, она (feminine). Therefore we use the verb in feminine: “была”, even though the Subject is plural (знакомые).

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