Learn Hardcore Korean: sajeoneul yeoreoseo moreuneun daneo tteuseul chatseupnida. - I open the dictionary to find the meanings of unknown words.

The -아서/어서 form links two verbs, showing that the second action follows from or is caused by the first. Here: “by opening the dictionary (사전을 열어서), (I) look up the meanings of words I don’t know (모르는 단어 뜻을 찾습니다).” It’s more than a simple “and”; it implies sequence or method.

How do you decide whether to use -아서 or -어서?

It depends on the final vowel of the verb stem before you attach it:
• If the stem ends in ㅏ or ㅗ, use -아서 (e.g. 가다 → 가서).
• For all other stems, use -어서 (e.g. 먹다 → 먹어서, 열다 → 열어서).

Why is it 모르는 단어 and not 모른 단어?

In Korean, to turn a verb into a present-tense modifier of a noun, you add -는 to its stem. So 모르다 (to not know) becomes 모르는 when describing 단어. If you wanted to express a past-tense modifier, you’d use -은 (e.g. 모른 적 “a time I didn’t know”).

Why is there no particle between 단어 and 뜻? Can I say 단어의 뜻?

Korean often forms noun-noun compounds without a particle. 단어 뜻 literally means “word meaning.” You absolutely can say 단어의 뜻 (adding the genitive ), which is slightly more formal or explicit. In everyday speech, omitting is very common.