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Questions & Answers about Кто-то открыл дверь.
How does -то change the meaning of кто here?
Adding -то turns the question word кто (who) into the indefinite pronoun кто-то (someone). It is used when an action definitely happened, but you do not know exactly who did it.
Why is the verb открыл masculine if we don't know the gender of this 'someone'?
Дверь is the direct object here. Why didn't its ending change?
Дверь (door) is a feminine noun that ends in a soft sign (-ь). For these types of feminine nouns, the accusative case (which we use for direct objects) looks exactly the same as the dictionary form. It stays дверь, unlike feminine words ending in -а which would change to -у.
If I needed to use кто-то as an object in another sentence, how does it change?
When declining words with -то, only the first part changes, while the hyphen and -то just tag along at the end. Here it is the subject (nominative), but if you wanted to say 'I saw someone' (accusative), you would change кто to кого and get кого-то.