N3
ずにはいられない
Cannot help doing something, Cannot resist doing something
美味しそうなステーキが目の前にあったら、食べずにはいられない。
If there is a delicious looking steak in front of me, there is no way I can resist eating it.
Explanation
「〜ずにはいられない」 means 'can't help doing'. It attaches to the verb's ない-stem (「する」 becomes 「せずには」). A feeling, impulse or instinct is too strong to hold back, so you do it without meaning to: 「その話を聞いて、笑わずにはいられなかった」 (Hearing that, I couldn't help laughing).
Mixed up with
Unlike 「〜ざるを得ない」: 「ずにはいられない」 comes from inside, you cannot stop yourself (laugh, cry); 「ざるを得ない」 is forced by outside circumstances, you do it though you don't want to (work overtime).
The subject is normally 'I'. For other people, add 「〜ようだ」, 「〜らしい」 and so on at the end.