Learn Russian: Чьи это новые часы?

The Russian word for watch or clock, часы, is always plural. It works just like the words 'glasses' or 'scissors' in English. Because the noun is plural, any adjectives or pronouns describing it must also be plural.

Where does the word чьи (whose) come from?

Чьи is the plural form of the question word чей (whose). Because часы is grammatically plural, we have to use the plural form чьи instead of the masculine, feminine, or neuter singular forms.

If everything else is plural, why is это used instead of the plural эти?

In this sentence, это is acting as a fixed pointing word that means 'this is' or 'these are.' When used to point something out or ask what something is, это stays frozen in this neuter singular form and does not change to match the noun.

How do we form the plural for the adjective новый (new) in this sentence?

The basic masculine form is новый (new). To make it plural to match часы, we replace the masculine ending with the plural ending -ые, which gives us новые. This is the standard plural ending for most adjectives in Russian.