Learn Swedish: Fågeln tittar på cykeln. - The bird looks at the bicycle.

Both fågel (bird) and cykel (bicycle) are en-words that end in an unstressed -el. When we make them definite to mean "the", Swedish drops the e from that -el ending before adding the -en suffix. So, fågel plus -en becomes fågeln, and cykel plus -en becomes cykeln. This stops the words from having too many weak vowel sounds in a row!

Since they just end in -n, how do I know if fågeln and cykeln are en-words or ett-words?

Even though you only see an -n at the very end, the suffix we added was actually -en (the standard suffix for en-words). The missing e was dropped from the root word itself (fågel becomes fågl-). If they were ett-words, we would have added -et, which would result in words like fåglet or cyklet.

Why is the word på used after tittar?

The verb tittar alone just means "looks" or "is watching". When you want to specify that someone is looking at something, you must use the two-word phrase tittar på.