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Questions & Answers about Мальчик трогает колено.
Why isn't there a word for "his" in the Russian sentence?
In Russian, possessive pronouns like своё (his/one's own) are usually left out when talking about body parts, clothing, or personal items if it is obvious who they belong to. Since the boy is doing the action, it is naturally assumed he is touching his own knee.
The boy is touching the knee, so "knee" is the direct object. Why doesn't колено change its ending?
You are right that direct objects take the accusative case. However, колено is a neuter noun because it ends in -о. Inanimate neuter nouns look exactly the same in the accusative case as they do in the nominative (dictionary) case.
What is the dictionary form of the verb трогает?
The dictionary form (infinitive) is трогать (to touch). It follows a very common and regular conjugation pattern, dropping the -ть and adding -ет for the he/she/it form.
Does трогает mean he is touching it right now, or that he touches it generally?
Because трогать is an imperfective verb, it can mean both "touches" (habitually) or "is touching" (right now). Since Russian doesn't have a separate continuous tense like English, we usually assume it means he is touching it right now unless there are other context words.