(something) is one thing, (something else) is (quite) another

While it may be acceptable to do something, that does not make it acceptable to do something else that is much more extreme or complicated than the first. Asking us to stay an extra hour each evening to finish a project is one thing, but demanding we work every weekend without overtime pay or else risk being fired is quite another. Expressing your affection for someone you've started dating is one thing; bombarding them with declarations of love after just a few days is another. Occasional sadness is one thing, but this prolonged, intense depression you've been experiencing is quite another.

it's one thing to (do something), it's another to (do something else)

While it may be acceptable to do something, that does not make it acceptable to do something that is much more extreme or complicated than the first thing. It's one thing asking us to stay an extra hour each evening to finish a project, but it's quite another to demand we work every weekend without overtime or else risk being fired. It's one thing to express your affection for someone you've started dating. It's another to bombard them with declarations of love after just a few days.

See also: another, one, something, thing, to

it's one thing to (do sth), it's another thing to (do sth else)

While it may be acceptable to do something, that does not make it acceptable to do something that is much more extreme or complicated than the first. It's one thing asking us to stay an extra hour each evening to finish a project, but it's another thing to demand we work every weekend without overtime pay or else risk being fired. It's one thing to express your affection for someone you've started dating; it's quite another thing to bombard them with declarations of love after just a few days.

it's one thing to (do sth), it's quite another to (do sth else)

While it may be acceptable to do something, that does not make it acceptable to do something that is much more extreme or complicated than the first. It's one thing asking us to stay an extra hour each evening to finish a project, but it's quite another to demand we work every weekend without overtime pay or else risk being fired. It's one thing to express your affection for someone you've started dating; it's quite another to bombard them with declarations of love after just a few days.

See also: another, one, quite, sth, thing, to

it’s ˌone thing to do ˈA, it’s (quite) aˌnother (thing) to do ˈB

A is ˈone thing, B is (quite) aˈnother

used for saying that you find the first thing acceptable or possible but the second thing definitely unacceptable or impossible: It’s one thing to write a short article; it’s quite another to write a whole book on the subject. ♢ Romance is one thing, marriage is quite another.