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Questions & Answers about Этот ботинок старый.
Is ботинок singular or plural here?
It is singular, meaning just one "boot." We usually talk about boots in the plural (ботинки), but here we are pointing out one specific boot. Be careful: ботинок is also the genitive plural form (the form used when saying "a lot of boots" or counting 5 or more), but in this sentence, it is just the basic masculine singular dictionary form.
Why do we use Этот instead of the generic Это?
Этот is the masculine form of "this" and must match the masculine noun ботинок. You use the generic word Это when "this" acts as the whole subject by itself, like in Это старый ботинок (This is an old boot). But because our sentence pairs "this" directly with the noun (This boot...), they must match in gender.
How would I change this sentence to say "These boots are old"?
You need to change all three words into their plural forms. Этот becomes Эти, ботинок becomes ботинки, and старый becomes старые. The full plural sentence is Эти ботинки старые.
Why does the adjective старый end in -ый instead of -ой?