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 의), which is slightly more formal or explicit. In everyday speech, omitting 의 is very common.

What tense and level of politeness is 찾습니다, and who is the subject?

찾습니다 is the present-simple, declarative, polite form (하십니다 style). Korean often drops the subject when it’s clear from context. Here the implied subject is “I” (or sometimes “we” in a general/habitual sense).

Could I use 찾아봅니다 instead of 찾습니다 here?

Yes.
• 뜻을 찾다 means “to find the meaning.”
• 뜻을 찾아보다 means “to look up the meaning,” emphasizing the searching process.
Both are correct: 사전을 열어서 모르는 단어 뜻을 찾아봅니다 is perfectly natural.

Can I replace 사전을 열어서 with 사전에서?

You could say 사전에서 모르는 단어 뜻을 찾습니다 to mean “(I) look up unknown word meanings in the dictionary.” However, 사전을 열어서 explicitly describes the act of opening the physical book first, highlighting the sequence “open → search.”

If I want to express this in the past tense, what changes?

Change the endings to the past polite form:
사전을 열어서 모르는 단어 뜻을 찾았습니다.
(The connector -어서 stays the same in past sequences.)