what's the difference between 痛い and 辛い?

Question

Online dictionaries give 痛い and 辛い the meaning "painful" and among others:

痛い (itai): sore

辛い (tsurai) bitter, heart-breaking

In what contexts one of them can be used while the other can't?

In addition, I've seen 辛い can also be used to mean "tough", "hard" and "stressful", while 痛い seems to be used for emotional and physical pain (?)

Any else I have to take note of?

Thanks

Answer 1

Before I get too far, you should know that つらい is usually written in kana alone, as 辛い can be read as either つらい or からい. Most of the time, it is read as からい with the kanji, so it can get confusing to the reader if you aren't careful.

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Short Answer:

痛い tends to be more clinical in definition. It is more often associated with physical pain, but is commonly used with emotional/mental pain as well. If it is part of your body, you will use 痛い. It will be used to specify the location of pain being experienced. As such, use of 痛い　is less subjective.

つらい is less tangible, and tends to be an exaggeration. I see it most commonly used to describe tasks or jobs that the speaker finds grueling or difficult. You will never see it used to describe pain to a doctor, the main exception being a psychiatrist. The meaning is more subjective.

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(Very) Long Answer:

With issues of this nature, I find it best to look at the Japanese definition, as it is a better look at how they use the word than the J/E dictionary. Sometimes meaning is lost in translation.

つらい：

１ 他人に対して冷酷である。非情である。むごい。「―・いしうち」「―・く当たる」
２ 精神的にも肉体的にも、がまんできないくらい苦しい。苦しさで耐えがたい。「―・い別れ」「いじめられて―・い目にあう」「練習が―・い」
３ 対処が難しい。困難である。「―・い立場にいる」「その話をされると―・い」
４ 人の気持ちを考えない。つれない。
「からころも君が心の―・ければ袂はかくぞそぼちつつのみ」〈源・末摘花〉
５ 冷たい態度が恨めしい。しゃくにさわる。
「―・しともまた恋しとも様々に思ふ事こそひまなかりけれ」〈和泉式部日記〉

Translation (bold parts only):

1. In regard to a person, ruthless. Cold hearted. Brutal (or cruel).
   
2. Physically or Emotionally painful to the point that it cannot be born. Unbearable suffering.
   
3. Difficult to cope with. Hardship.
   

(I skipped the definitions that aren't super related to the question.)

I usually heard it in context of #2, where they would say things like, 仕事はつらい。 or with respect to some sort of activity that required a lot of effort. Typically, it was used as a slight over exaggeration. It reflects the feelings of the speaker that what they were doing was physically or emotionally difficult.

痛い：

１ 肉体に痛みや苦しみを感じるさま。「歯が―・い」「つねられて―・い」
２ 心に苦痛を感じるさま。精神的につらい。「欠損続きで頭が―・い」
３ 弱点を攻撃されたり打撃や損害をこうむったりして、閉口するさま。「―・いところに触れられる」「―・い目にあう」「この時期に出費は―・い」
４ 俗に、さも得意そうな言動がひどく場違いで、見るに堪えないさま。また、状況や立場・年齢にふさわしくない言動が周囲をあきれさせるさま。
５ （甚い）程度のはなはだしいさま。多く、連用形を用いる。→甚 (いた) く
６ （甚い）はなはだしくりっぱなさま。すばらしい。
「新発意 (しぼち) の娘かしづきたる家いと―・しかし」〈源・若紫〉
７ 動詞の連用形に付いて、その動詞の表す状態がはなはだしい意味を示す形容詞をつくる。「あまえいたし」「うもれいたし」など。

Translation (bold parts only):

1. Appearing to experience physical pain or suffering.
   
2. Appearing to experience pain/agony in the heart (i.e. your emotional core). Suffering mentally.

Ironically, they use つらい in the second definition. I likewise skipped definitions I didn't feel directly apply to the question.

痛い has more of a clinical feel to it. As such, it is less subjective. This is because the kanji radical for 痛い (疒) is associated with sickness. There tends to be less exaggeration than you see with つらい, but you cannot rule out exaggeration completely. I most often heard it used as (body part)が痛い. People also use 痛い the same way English users say ouch! Also, when referring to emotional/mental pain, the use is also more clinical.

Answer 2

TL;DR: つらい (as ajsmart's answer indicates, it's very confusing if you use the Kanji for this word; in fact I had a double-take when I read the question because I thought "spicy and pain are completely different, what?!") is usually more describing emotional pain, while 痛い is more about describing physical pain. For example, you could say:

今日、上司に怒られて、怒鳴られた。つらかったよ～！
Today, my boss was really mad at me, and he yelled at me. I was really hurt!

or you can say:

今日起きたら、腕が痛くて動けなかった
Today, when I woke up, my arm hurt and I couldn't move it.

You could not swap 痛い and つらい in those 2 sentences, it would be super weird.

Edit: Actually you could use つらい in the second sentence, like:

今日起きたら、腕が動けなくてつらかった！

This would be something similar to "Today when I woke up, I couldn't move my arm, and it really sucked". In this context, つらい translates roughly as the English slang phrase "it sucks".