Learn Romanian: Femeia are patru copii. - The woman has four children.

No, it stays exactly the same. In Romanian, only the numbers one (unu/o) and two (doi/două) change to match the gender of the noun. The number four is always patru regardless of what you are counting.

Why does the word copii have two i's at the end?

The singular word for child is copil. When making masculine nouns that end in -l plural, the -l is often dropped and replaced with an -i. This creates the double -ii ending in copii. You pronounce it by holding the "ee" sound a little longer than you would for a single i.

Where is the word for "the" in Femeia?

In Romanian, the word for "the" (the definite article) attaches directly to the end of the noun instead of being a separate word. The basic word for woman is femeie. To say "the woman", the final -e changes to -a, making it femeia.

Why is the verb are used here instead of am or ai?

Are is the "he, she, or it" form of the verb a avea (to have). Because "the woman" is a "she" (ea), we must use are. You would use am for "I have" and ai for "you have".