Learn Swedish: Jag älskar tomater.

The singular is en tomat. It belongs to Group 3 nouns, which form their plural by adding -er (or sometimes just -r if they already end in a vowel). Since tomat ends in a consonant, we simply add -er to make tomater.

Is there a trick to remember which words take this -er ending?

Yes! A huge number of en words that end in a consonant belong to Group 3 and take the -er ending. Words that Swedish borrowed from other languages, like en tomat or en telefon, almost always follow this exact pattern.

Why do we use the indefinite plural tomater here instead of the definite tomaterna?

Just like in English, when you talk about liking or loving things in general, you use the indefinite plural. You say "I love tomatoes" rather than "I love the tomatoes", so in Swedish we say Jag älskar tomater.

Would the verb älskar change if multiple people loved tomatoes?

No, Swedish verbs are very friendly! They never change based on who or how many people are doing the action. Whether the subject is jag (I), hon (she), or vi (we), the verb stays exactly the same: älskar.