Learn Romanian: Copilul mănâncă struguri.

In Romanian, the word for "the" (the definite article) attaches to the end of the noun. The basic word for "child" is copil. Because it is a masculine noun ending in a consonant, we add -ul to the end to make copilul ("the child").

What is the dictionary form of mănâncă, and why does the root look different here?

The verb is a mânca (to eat). When conjugated for "he/she/it" (like copilul), the spelling of the root shifts slightly from mânc- to mănânc-, and we add the regular ending -ă. These kinds of vowel changes inside the word are very common in Romanian verbs.

Why doesn't the sentence use niște (some) before the grapes?

You could say Copilul mănâncă niște struguri, which specifically means "The child is eating some grapes." However, just like in English, if you are making a general statement about the type of food being eaten, you can drop the article and just use the bare plural noun struguri (grapes).

What is the singular form of struguri?

The singular is strugure (a grape). Even though it ends in -e, which might make it look feminine, strugure is a masculine noun. To make it plural, the ending changes from -e to -i, giving us struguri.