I have a question about describing my house. I know the word is viejo for old, nuevo for new and modern is moderno. If I say Mi casa is modern do I say Mi casa is modena by changing the "o" to the feminie "a". The same for viejo if I describe my house would I change the "O" to an "a" in viejo like Mi casa es vieja.

yes, adjectives must agree with their subjects. Since la casa is feminine, the adjectives must change o to a: nueva, vieja

Yes, but moderna, not modena

Great question! In Spanish, adjectives usually agree in gender and number with the noun they modify. So, when describing a feminine noun like "casa" (house), you would indeed change the ending of the adjective to match the feminine form.

For example, if you want to say "My house is modern," you can say "Mi casa es moderna." Here, "moderna" is the feminine form of the adjective "moderno."

Similarly, if you want to describe your house as old, you would say "Mi casa es vieja." In this case, "vieja" is the feminine form of the adjective "viejo."

Remember that it's important to match the gender of the noun you're describing with the gender of the adjective. This rule applies not only to "casa" but to all feminine nouns you describe.