Learn German: Ein Millionär hätte eine große Villa. - A millionaire would have a large villa.

hätte is the Konjunktiv II (Subjunctive II) form of haben. While hat means 'has' (describing a real fact), hätte translates to 'would have' (describing an imagined or hypothetical situation).

What is the difference between hatte and hätte?

The umlaut (the two dots over the a) makes a huge difference! hatte is the simple past tense meaning 'had' (a real past event). hätte is the Konjunktiv II meaning 'would have' (an imaginary situation).

Why is it eine große Villa with an -e on the adjective?

Villa is a feminine noun. In this sentence, it is the direct object (the thing being had), so it is in the accusative case. After the feminine article eine, the adjective groß needs the -e ending to match.

How would I change this sentence if the millionaire were a woman?

You would say Eine Millionärin hätte eine große Villa. To talk about a female millionaire, you add the -in suffix to make the noun feminine, which also changes the word for 'a' from ein to eine.