I'm confused about ' 中 '

[PrismPowerr]

Whenever '中' appears, the voice over says, "Naka" but I am expected to choose 'ちゅう' in the multiple choice.

It's confusing, because it should just say 'なか' to match the voice over. It's also confusing, because when its '中国' THEN it uses the 'ちゅう' sound, but when it's after a sound, like '田中' then it remains 'Naka'.

Is there some rule that in the beginning of the word, its 'ちゅう' at the end, its 'naka'? Or are these two words exceptions, and the word is actually one or the other?
For now I'm just memorizing the words it's associated with and expecting either, but not knowing when or why it will be one or the other. Σ(-᷅_-᷄๑;)

[Arachnje]

As antiukap mentioned, there are many exceptions to the rule, and here are some common examples:

Compound words with kunyomi reading

甘口（あまくち): sweet taste
旅先　（たびさき): destination
花嫁　（はなよめ）: bride
合間　(あいま): interval (time)
外側　（そとがわ): exterior (outer side)

Standalone words with onyomi reading

本　（ほん): book
茶　（ちゃ): tea
肉　（にく): meat

Compound words with joined kunyomi-onyomi readings

赤字　（赤字): deficit (financial)
寝坊　（ねぼう): oversleeping
甘党　（あまとう): sweet tooth (liking sweets)

Irregular reading

明後日　（あさって): the day after tomorrow
大人　（おとな): adult
田舎　（いなか): country side, rural area
今日　（きょう): today

Since exceptions are as common as rules, the only way to know the correct reading is to familiarize yourself with all possible readings for each kanji. It may seem to take time, but you can learn easily if you study kanjis grouped by their level, and I recommend grouping kanji according to their JLPT (Japanese Languages Proficiency Test) levels, from N5 (easiest) to N1 (most difficult)

The two resources below may help you out (click on the gray text))

Kanji list (1)

Kanji list (2): click on the kanji to get a pop-up card with explanation and on/kun reading

Additionally, you can use the The Yamasa Institute online dictionary for searching kanji by meaning or on/kun reading.

[antiukap]

Kanji can have several readings. Generally, they are divided into two categories: 音読み(onyomi) - readings "based" on Chinese pronunciation and 訓読み(kunyomi) - native Japanese words assigned to a character.

Kunyomi mostly used for standalone characters and onyomi - for compound words. But there are many exceptions.

For kanji '中': 'ちゅう' is on-reading (中学 - middle school) and 'なか' - kun-reading (車の中 - inside of a car).

Here is a video explaining kanji: https://youtu.be/CF3MRMBjd20

[Art_Heljin]

Japanese is not a good language to learn only on Duolingo: there's too much about kanji usage and such that Duo just can't give you, especially since it's in beta. I recommend finding some books on Japanese to read along with studying here.

[MaynardHogg]

Yep. Outside reading always helps.

DL is at best a drill program written by amateurs and without proofreaders or fact checkers.

[PrismPowerr]

Thanks everyone!

Antiukap thank you! And thanks for the link- I didn't know about Onyomi and Kunyomi!
Thank you Arachnje for the list!
Art- I actually am learning on other apps along with this one, but I'll look into this topic more specifically as its something I'm confused with.