Questions & Answers about Мама трогает ухо ребёнка.
Why doesn't the word for ear (ухо) change its ending here since it's the object being touched?
It actually is in the accusative case, which marks the direct object of the verb. However, ухо is a neuter noun. For neuter nouns, the accusative case looks exactly the same as the nominative (dictionary) form.
What case is ребёнка and why is it used here?
It is in the genitive case, which is used to show possession. You can think of it as meaning "of the child". In Russian, possession is shown by putting the owner in the genitive case right after the object they own.
Why is the spelling ребёнка instead of ребёнока?
The dictionary form for child is ребёнок. When adding case endings, the last о drops out. This is called a "fleeting vowel," and it is very common in Russian masculine nouns ending in -ок or -ец to make them easier to pronounce.
How does the verb трогает conjugate?
The infinitive is трогать (to touch). It follows the regular first conjugation pattern. Because мама acts as "she" (third-person singular), you drop the -ть from the infinitive and add -ет to get трогает.