Liking, being good at and understanding take が, not を — in Japanese these are states, not actions, and the thing liked is grammatically the subject.
Also known assuki desujouzuwakarimasu好き 上手ga particle likes
The object of 好き (like), 嫌い (dislike), 上手 (good at), 下手 (bad at) is marked with が.
わたしは にほんごが すきです。
I like Japanese.
These are adjectives of feeling/ability. What you like or are good at is marked with が, not を: ［person］は ［thing］が 好きです.
In the textbooksMinna I, lesson 9Genki I, lesson 5
わかります (understand) and あります (have) also take が for their object.
ミラーさんは にほんごが わかります。
Mr Miller understands Japanese.
With わかります (understand) and あります (have/possess), the thing understood or possessed is marked with が.
In the textbooksMinna I, lesson 9