If you haven’t started already, learning grammar really helps with the vocab. Some have recommended not starting grammar until level 10, but it does help to break down vocabulary and make it easier to remember.
For example, 大した is really 大 (big) plus the past tense of する (to do). So, “did a big thing” helps me to remember that this is an expression meaning “great”.
The other important point is that it’s very helpful to know when something is compound kanji (地理 - geography) or a compound word (大空 - heavens, basically the words for big and sky smooshed together). AFAIK, there’s no real pattern to when the kun or on readings are used other than going through the vocab.


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