Learn Russian: Мама моет шею ребёнку.

In Russian, when an action is performed on someone's body part (like washing, brushing, or hurting), the person is usually put in the dative case as the receiver of the action. It translates literally as "Mom washes the neck to the child." Using the genitive (ребёнка) for possession is possible, but the dative is the most natural and common way to say this.

Why is the word for neck spelled шею here instead of шея?

The dictionary form is шея (neck), which is a feminine noun. Because the neck is the direct object receiving the action of being washed, it must be in the accusative case. For feminine nouns ending in -я, the ending changes to -ю in the accusative case.

What is the infinitive form of моет? The spelling looks very different.

The infinitive is мыть (to wash). This is an irregular verb where the root vowel -ы- changes to -о- in the present tense. Adding the standard third-person singular ending gives us моет (she washes).

What happened to the letter -о- from the word ребёнок when it became ребёнку?

The word ребёнок (child) has what is called a "fleeting vowel." When you add a case ending like the dative -у, the -о- in the final syllable disappears to make the word easier to pronounce. That is why it becomes ребёнку and not "ребёноку".