Learn French: L'ordinateur n'est pas rapide.

The negative word ne drops its 'e' and uses an apostrophe because the verb est starts with a vowel. This is a mandatory rule in French to make pronunciation smoother.

How does the negative sandwich work here?

Basic French negation uses two words: ne (or n') and pas. They wrap around the conjugated verb. Here, they go on either side of est (is) to create n'est pas (is not).

Why does the sentence start with L'ordinateur instead of Le ordinateur?

Just like the negative n', the word for the (le or la) drops its vowel and takes an apostrophe when the next word starts with a vowel. Because ordinateur starts with an 'o', we use L'.

Would the adjective rapide change if the noun was feminine?

No. Usually, you add an 'e' to the end of a French adjective to make it feminine. However, because rapide already ends in an 'e' in its masculine form, the spelling stays exactly the same for feminine nouns.