Only as hard as "apple" and "apples" in English :)
Äpfel sounds almost like "Epfel" would.
Apfel - /ˈapfəl/ as in father
Äpfel - /ˈɛpfəl/ as in bedding
Plus, Apfel is a countable noun and so requires an article in the singular. The plural doesn't, like in English. The definite article also changes in the plural.
Ich will einen Apfel.
Ich will Äpfel.
I want an apple / I want apples.
Schau dir den Apfel an!
Schau dir die Äpfel an!
Look at the apple(s)!
But yes, they do look very similar, and they might be able to be mixed up without context.
Well, lucky day, because now you know it. Duolingo likes to "introduce" words to you rather than formally teach them to you. If you don't know a word, it should be underlined in orange (as long as you aren't testing). Hover over or tap it to see a list of definitions.
mögen (to like)
ich mag / du magst / er mag
wir mögen / ihr mögt / sie mögen
Alternatively, you can visit the Tips and Notes over at https://www.duolingo.com/skill/de/Verbs%3A-Present-1/tips-and-notes or by clicking the lightbulb option next to "Start Lesson" on the desktop.
If you're running windows 10, there is an English-international keyboard that adds umlaut shortcuts without changing the general layout of the keyboard. I found it very handy to enable, that way I don't have to memorize alt codes or use the Duo accent buttons to input the keys.