In Spanish, adjectives that modify a noun must agree in number and gender with the nouns they modify. This is true in English too. Here is an example:
The house is small
Notices that house and small are both singular. In Spanish, this is easier to understand:
La casa es pequeña - the house is small (casa and pequeña are both singular)
Las casas son pequeñas - The houses are small (notice that casas and pequeñas are both plural; so too are las and son for that matter, but we are confining this conversation to nouns and adjectives)
This concept may seem basic, but it's an easy one to forget or skip over. And it gets more difficult when the noun you use doesn't end in "o" or "a". Take for example:
El párque es grande - The park is large
Los párques son grandes - The parks are large (Noun: párque and adjective: grandes are both plural)
The noun does not end in "o" or "a", so determining gender can be a bit more difficult, but the adjective must still agree in number and gender with the noun it modifies.
This is where we chat about all things Spanish. Feel free to ask a question, challenge me, passionately disagree, or rant about whatever. Just make it interesting.