Learn Spanish (Latin America): Necesito un cepillo tuyo; lo quiero usar ahora. - I need a brush of yours; I want to use it now.

Tu cepillo means "your brush" (referring to a specific one), while un cepillo tuyo means "a brush of yours" (an unspecified one). When you use an indefinite article like un or una, the possessive adjective goes after the noun and takes its long form (tuyo, mío, etc.).

Does tuyo need to match the noun in gender and number?

Yes. Unlike the short form tu which only changes for number (tu vs tus), the long form changes for both gender and number. Since cepillo is masculine and singular, you use tuyo. If you needed a towel, it would be una toalla tuya.

Can I attach lo to the end of the verb instead of putting it before quiero?

Exactly! You can say quiero usarlo. When a conjugated verb works together with an infinitive (like quiero usar), you can put the pronoun (lo) either before the conjugated verb or attached directly to the end of the infinitive. Both are perfectly natural.

How do I know whether to use lo or la for "it" here?