В моей ручке закончились чернила, поэтому я не смог написать заголовок.

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Breakdown of В моей ручке закончились чернила, поэтому я не смог написать заголовок.

• мой: my
• в: in
• не: not

Questions & Answers about В моей ручке закончились чернила, поэтому я не смог написать заголовок.

Why is it в моей ручке? Why do моей and ручке have those endings?

Because в here means in and shows location, so it normally takes the prepositional case.

• ручка = pen
• Prepositional singular: в ручке = in the pen
• моя = my for a feminine noun
• In the prepositional case, моя becomes моей

So:

• в моей ручке = in my pen

Both the noun and the possessive word have to match in gender, number, and case.

Why is чернила plural if English says ink?

In Russian, чернила is usually a plural-only noun when it means ink.

So even though English treats ink as singular, Russian treats чернила grammatically as plural:

• чернила закончились = literally the inks finished
• natural English: the ink ran out

This is just one of those noun patterns you have to learn. Russian has several nouns that are grammatically plural even when English uses a singular mass noun.

Why is the verb закончились plural?