Learn Russian: У неё болят суставы, потому что это хроническая болезнь. - Her joints hurt because it is a chronic illness.

In the Russian physical pain construction, the body part that hurts is actually the grammatical subject of the sentence. Because суставы (joints) is plural, the verb болеть (to hurt) must take the plural form болят. If only one joint were hurting, you would use the singular болит.

Why is it у неё with an "н" instead of just у её?

Whenever a third-person pronoun (like он, она, оно, or они) comes directly after a preposition, Russian adds an н- to the beginning of it to make it easier to pronounce. So, the genitive form её (her) becomes неё after the preposition у.

Болезнь (illness) is feminine, so why does the second half of the sentence use это instead of она?

We use the invariable word это when we are introducing, identifying, or defining what something is (meaning "This is a..." or "It is a..."). Even though the noun being identified (болезнь) is feminine, это stays in its neutral form when it acts as this kind of "pointing" word.

Since болезнь ends in a soft sign (-ь), how can I tell what gender it is?