English has fixed phrases for placing something on a timeline. In the long run means eventually, once enough time has passed; it is neutral in register, so it works in both writing and everyday speech. In the short term is its opposite: it means soon, or right now, and it is equally neutral. Both words work in both phrases, in the long run / in the short run, in the long term / in the short term, just don't cross a pair inside one sentence. Keep the meanings straight: the long one points to the far-off payoff, the short one to what happens immediately. When time seems to pass quickly, we say time flies; this one is informal and conversational, a warm reaction to how fast the hours or years went. The words are locked, so you can't swap or reorder them.
How soon, and how far ahead
in the long run /ɪn ðə lɔŋ rʌn/ — Cutting corners now will cost us more in the long run.
in the short term /ɪn ðə ʃɔrt tɜrm/ — In the short term the new system feels slower, but people adjust.
time flies /taɪm flaɪz/ — I can't believe she's already ten; time flies.
Not ""The diet is hard now, but in the short term you'll be much healthier."" — say ""The diet is hard now, but in the long run you'll be much healthier."". 'In the short term' means soon or right now, so it clashes with a benefit that only arrives after a long time. When the payoff comes eventually, you need in the long run.
Try it: Which phrase best completes: "It's expensive to train staff, but ___ it saves money."? (in the long run / in the short term / time flies) Answer: in the long run — The sentence contrasts a cost now with a benefit later, so you want 'in the long run', meaning eventually. 'In the short term' would mean soon, which reverses the contrast. 'Time flies' is about time passing quickly, not about when a benefit lands.
Waiting and staying flexible
Some idioms are about patience rather than clock time. Play it by ear means to decide as you go instead of planning ahead; it is informal and spoken, at home in relaxed, everyday plans. Bide your time means to wait patiently for the right moment to act; it is neutral in register and carries a deliberate, strategic feel — you are holding back on purpose, not just drifting. Notice the contrast: play it by ear is staying flexible with no plan, while bide your time is waiting with a clear goal in mind. Once in a blue moon means very rarely; it is informal and conversational. Again, the words never change.
play it by ear /pleɪ ɪt baɪ ɪr/ — We haven't booked anything; let's just play it by ear.
bide your time /baɪd jʊr taɪm/ — She bided her time and asked for a raise once the project succeeded.
once in a blue moon /wʌns ɪn ə blu mun/ — He only calls once in a blue moon, so it was a surprise.
Not ""We have no plans yet, so we'll bide our time this weekend."" — say ""We have no plans yet, so we'll play it by ear this weekend."". 'Bide your time' means waiting patiently for one right moment to act, so it needs a goal you're holding out for. With no plan and a relaxed, decide-as-you-go weekend, the idiom is play it by ear.
Try it: Which phrase best completes: "We don't have a fixed plan for the weekend, so we'll just ___."? (play it by ear / bide our time / once in a blue moon) Answer: play it by ear — No fixed plan and deciding as you go is exactly 'play it by ear'. 'Bide our time' means waiting patiently for one right moment, which doesn't fit a relaxed weekend. 'Once in a blue moon' means rarely, and it can't complete this clause grammatically.
Try it: Which sentence uses its idiom correctly? (I see my old classmates once in a blue moon. / I see my old classmates once in a blue sky. / I see my old classmates in a blue moon once.) Answer: I see my old classmates once in a blue moon. — The fixed idiom for 'very rarely' is 'once in a blue moon'. 'Once in a blue sky' swaps 'moon' for 'sky', which breaks the phrase. 'In a blue moon once' reorders the words, and time idioms can't be reordered.