Learn Italian: Quest'albero è vecchio, ma questa casa è nuova. - This tree is old, but this house is new.

The word questo (this) drops its final -o and takes an apostrophe when the noun right after it begins with a vowel. Since albero starts with an a, questo albero shrinks to quest'albero to make it flow better when spoken.

Why do we use questa for the house but not for the tree?

Words for "this" must match the gender of the noun they describe. Casa (house) is a feminine noun, so we use the feminine form questa. Albero (tree) is masculine, so we use the masculine questo (which becomes quest' here).

If a feminine noun started with a vowel, would it also use quest'?

Yes! Just like questo, the feminine questa drops its final -a before another vowel. For example, "this island" (questa isola) becomes quest'isola. When you see quest', you have to look at the noun it is attached to in order to know if it is masculine or feminine.

Why do the adjectives end differently (vecchio vs. nuova)?