Questions & Answers about Я поставил свой стул между обоими окнами.
Why do we use обоими in this sentence?
The preposition между (between) requires the Instrumental case. The noun окно (window) is neuter, so the word for 'both' is based on the masculine/neuter oblique root обо-. Adding the Instrumental ending gives us обоими.
Wait, doesn't 'both' have a special form for feminine words? Is окно feminine?
Окно is a neuter noun. In Russian, masculine and neuter nouns share the same forms for 'both' (using the root обо- in oblique cases). The feminine root обе- is strictly reserved for feminine nouns, so it is not used here.
Could I say мой стул instead of свой стул?
You would be understood, but свой is the much more natural choice here. Whenever the subject of the sentence is acting upon their own possession, Russian strongly prefers the reflexive possessive pronoun свой rather than a standard pronoun like мой.
Why use the verb поставил to mean 'put'?
In Russian, verbs of placing depend on the object's physical orientation. You use поставить (to stand something up) for objects that have legs or a clear base, like a chair (стул). If you were placing something flat on the ground, you would use a different verb, like положить.