Questions & Answers about О чём ты думаешь?
Why do we use о here instead of об?
The preposition meaning "about" changes to make pronunciation easier. We use об before words starting with a vowel (like об Анне - about Anna) and о before words starting with a consonant. Since чём starts with the consonant ч, we use о.
Where does the word чём come from?
Чём is the prepositional case form of что (what). When we talk about something using the preposition о, the noun or pronoun that follows must be in the prepositional case. So, "about what" always becomes о чём.
In English, we usually put "about" at the end of the sentence ("What are you thinking about?"). Why is it at the beginning here?
In Russian, prepositions must stay directly in front of the word they belong to. You can never leave a preposition hanging at the end of a sentence like in English. This sentence is structured exactly like "About what are you thinking?".
Why does the verb end in -ешь?
The ending -ешь tells us that the action is being done by ты (you, informal). The dictionary form of the verb is думать (to think). To make it match ты, we drop the -ть and add -ешь.
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