～ては／では
if / when (and so)
～ては is the て-form of a verb or adjective plus the particle は. It becomes では after nouns and な-adjectives, and after voiced て-forms such as 飲んでは. The は turns the action into a condition that is being judged: “if/whenever X happens, then ...”, and what follows is usually something bad.
1. Prohibition: ～てはいけない
The most common use. Literally it means “if you do X, it won't do”, which is how Japanese says “you must not do X”.
ここでタバコを吸ってはいけない。
“You must not smoke here.”
- ここ = here
- で = place-of-action marker
- タバコ = cigarette
- を = object marker
- 吸っては（すっては）= if (you) smoke (吸う, to smoke)
- いけない = it won't do / it is not allowed
2. A condition with a bad result
Outside the fixed prohibition, ～ては connects a condition to an undesirable conclusion: because of X, something good becomes impossible.
そんなに高くては、買えません。
“If it's that expensive, I can't buy it.”
- そんなに = that much, so
- 高くては（たかくては）= if it is (that) expensive (高い, expensive)
- 買えません（かえません）= cannot buy (potential of 買う, to buy)
How to form it
Verb／い-adjective て-form + は; noun／な-adjective + では
- 吸う（すう）= to smoke → 吸っては（すっては）= if (you) smoke
- 飲む（のむ）= to drink → 飲んでは（のんでは）= if (you) drink
- 高い（たかい）= expensive → 高くては（たかくては）= if it is expensive
Key idea
～ては frames an action as a condition and warns about where it leads: “do X, and (something bad) follows”. Add いけない and you get Japanese's standard “must not”.
Built on the same stem
Updated .