Learn Romanian: Casa nu este foarte mare. - The house is not very big.

Yes, making a sentence negative is very straightforward in Romanian! You simply put nu (not) right before the verb. In this sentence, it goes right before este (is) to make nu este (is not).

I learned that house is casă. Why is it spelled casa here?

In Romanian, the word for the (the definite article) attaches to the end of the noun. For feminine words ending in -ă like casă, you drop the -ă and add an -a. That turns casă (a house) into casa (the house).

Does the adjective mare change because casa is a feminine noun?

No, mare is an exception! While most Romanian adjectives change their endings to match masculine or feminine nouns, adjectives ending in -e in their dictionary form (like mare) are the exact same for both masculine and feminine singular nouns.

What is the rule for placing words like foarte (very)?

Foarte works exactly like very does in English. It always goes directly before the adjective it is describing. That is why it is foarte mare (very big).