Difference between V-려고 하다 and V-기로 하다, to plan something

Wondered about the difference between V-려고 하다 and V-기로 하다 when they mean "to plan something"? Look no further!

This post focuses on the two constructs when they mean "to plan something."

Simply put, V-(으)려고 했다 (했어요) means "to have planned something (but the plan did not go through)." The idea of "intention" is communicated with this pattern.
Simply put, V-기로 했다 (했어요) also means "to have planned something." The idea of "decision" is communicated with this pattern.

Both expressions are most often used with 하다 conjugated in the past tense, so V-(으)려고 했다 and V-기로 했다. However, V-(으)려고 했다 means the plan has not been completed, were canceled, abandoned, etc.; because of that it is often followed by -ㄴ/는데 or -지만 (V-(으)려고 했는데 / V-(으)려고 했지만).

V-기로 했다 is used for plans regardless of completion. V-기로 했다 does not have a negative connotation, whereas V-(으)려고 했다 does. However, plans already completed will usually be expressed with the past tense, V-았/었다.

Examples:

1) 친구들이랑 점심을 먹으려고 했어요.
I planned to eat lunch with my friends (but, depending on the context, did not eat it or canceled it).

2) 친구들이랑 점심을 먹기로 했어요.
I planned to eat lunch with my friends (and have not had lunch with my friends yet, i.e. the plan is upcoming, is not canceled).

2bis) 친구들이랑 점심을 먹었어요.
I ate lunch with my friends.

However, when negative propositions are used:

3) 친구들이랑 점심을 먹으려고 했는데 날씨 때문에 취소가 됐어요.
I planned to eat lunch with my friends but, because of the weather, it got canceled.

4) 친구들이랑 점심을 먹기로 했는데 날씨 때문에 취소가 됐어요.
I planned to eat lunch with my friends but, because of the weather, it got canceled.

While sentences 3 and 4 can be translated the same in English, there is a very slight nuance in Korean. 3 implies that you really tried to make it work but it got canceled eventually, whereas in 4 this nuance is not present.

Words like "진짜" can emphasize this nuance, and can impact the English translation of the sentence:

5) 친구들을 만나려고 백화점에 진짜 가려고 했는데 못갔어요.
I really tried to go to the department store in order to (=with the intention of) meet my friends but I could not go.

6) 친구들을 만나려고 백화점에 가기로 했는데 못갔어요.
I planned to go to the department store to meet my friends but I could not go.
Caution! The first -(으)려고 in this example is not followed by 하다 and just means "with the intention of."

7) 비행기가 7시에 착륙하려고 했는데 못했어요. (X)
V-려고 하다 (regardless of the tense) is not used when the subject is not a person. -려고 하다 (regardless of the tense) communicates an intention, and therefore can only be used for people.

8) 비행기가 7시에 착륙하기로 했는데 못했어요. (O)
V-기로 했다 can be used regardless of the subject.

Other examples:

9) 공원에 가려고 했는데 못갔어요.

10) 공원에 가기로 했는데 못갔어요.

Notes:

Depending on the context and/or intonation of the speaker, other ideas may be conveyed. For example, 친구들이랑 점심을 먹으려고 했는데 감기가 걸려서 못갔어요 may also convey sadness that you were not able to eat with your friends because you caught a cold. These small nuances will be understood best with practice.
While most of the examples in this post are complex sentences with ㄴ/는데 and -지만, V-기로 하다 can always be used for simpler sentences where only the idea of planning needs to be conveyed. For example: 친구들을 만나려고 백화점에 가기로 했어요 (= I planned to go to the department store to meet my friends; does not imply that you will not go or could not go).
Other examples of the two patterns can be found in the grammar book Korean: a Comprehensive Grammar (read its KoniKorean review).