Learn Romanian: Această insectă este foarte mare.

Both mean "this insect," but their placement changes the noun's form and the sentence's tone. When the demonstrative comes before the noun (această), the noun doesn't take a definite article. This word order sounds slightly more formal or emphatic. If it comes after, the noun needs the definite article (insecta aceasta) and sounds more like everyday spoken Romanian.

Insectă is a feminine noun. Why is the adjective mare instead of having a feminine ending like mară?

The adjective mare belongs to a group of Romanian adjectives that end in -e. These adjectives have the exact same form for both masculine and feminine nouns in the singular. So, it is un câine mare (a big dog - masculine) and o insectă mare (a big insect - feminine).

How would this entire sentence change if we were talking about multiple bugs ("These insects are very big")?

It would become Aceste insecte sunt foarte mari. Notice that everything except foarte (which is an adverb and never changes) has to agree in the plural. Această becomes aceste, insectă becomes insecte, este becomes sunt, and the singular adjective mare becomes mari in the plural.