Learn Swedish: Jag behöver gaffeln till fisken.

The basic word for 'a fork' is en gaffel. Because it ends in an unstressed -el, Swedish drops that last e when adding the definite suffix (the ending that means 'the'). So instead of gaffelen, it shrinks down to gaffeln.

Why does fisken get the full -en ending, while gaffeln only gets -n?

Both are en-words. For standard en-words ending in a consonant like fisk (fish), we add the full -en to mean 'the', making it fisken. But because gaffel ends in an unstressed -el, it follows the special spelling rule: it drops the e and just takes an -n to become gaffeln.

The translation says 'for the fish', but isn't till usually translated as 'to'?

Yes, till often means 'to' (like going to a physical place). However, Swedish commonly uses till when talking about items that belong together or are meant for a specific purpose. You use till to describe a fork 'for' the fish, a charger 'for' a phone, or a key 'to' a door.

What is the base dictionary form of the verb behöver?