Omae Dattanda (おまえだったんだ)

You (subject) beside at am, you (subject) is-laughing

お前 ('you, rough/familiar male') is one notch below 君 in formality and is used between male friends, by older to younger, by men to spouses. Common in shounen-style male friendship songs.

You (subject) happy-look-and, I also laughed

Adjective stem + そう (looks like) for visible feelings of OTHER people: 嬉しそう ('looks happy'), 楽しそう ('looks fun'), 悲しそう ('looks sad'). For your own emotion, just use the bare adjective: 嬉しい ('I'm happy').

You (topic) innocent was

無邪気 (literally 'no evil-spirit') means innocent, child-like, naive in a positive sense. The song uses it for an old friend who, before adulthood, was uncomplicated.

You (topic) kind-was

い-adjective past tense: 優しい → 優しかった (drop い + かった). 優しい is one of the most common adjectives, covering 'kind, gentle, easy, mild' depending on context.

You (topic) sometimes cry, I (topic) was-troubled

困る (literally 'be in difficulty') has a wider range than English 'troubled' — it covers being at a loss, being in a fix, being annoyed. 困った covers all the 'I-don't-know-what-to-do' moments.

Anyone than strength (object) sought that-time, my only-one's weakness

誰より ('more than anyone') with comparative より is the standard 'most-X-of-all' construction. たった一つ ('just one, only one') uses たった as an emphatic 'merely / only', stressing scarcity.

You was (it.is)

〜だったんだ ('it was X (now I see)') is the past-tense explanatory ending — used for realizations and revelations. Title-line of the song: 'It was you (the one I lost, the one I'm calling back).'

Want-to-meet, night (object) cross, time (object) cross, now want-to-meet

Verb-stem + 越える ('cross over / surpass') is used metaphorically for spans of time and difficulty. 夜を越える ('cross the night'), 困難を越える ('overcome difficulties').

You (subject) pride was

You (subject) all was because

I as-for, very could-not-forgive

とても + negative gives 'absolutely not / can't possibly': とても許せない ('can't possibly forgive'), とても無理 ('completely impossible'). Different from positive とても ('very') — context decides.

Asphalt on, was-illuminated shadow (topic)

Passive 照らされた ('was illuminated') with the agent left unstated — the streetlight is implied. The shadow on asphalt is a stock urban-loneliness image, drawing on the stark contrast of high-pressure city sodium lights.

Alone, setback (object) was-repeating, that-time

挫折 ('setback / failure / frustration') comes from 挫く ('crush, frustrate') + 折る ('break'). Used for serious emotional defeats — failed exams, broken dreams. Heavier than 失敗 ('mistake').

Certainly you (subject) was-there

確かに ('certainly / indeed / for sure') is one of the most-used Japanese adverbs of confirmation. Different from たぶん ('probably'): 確かに is positive, certain.