What it means
ていては expresses a negative consequence of continuing in a certain state or action. It often corresponds to “if one keeps doing...” or “if things remain this way...” and suggests that the result is undesirable, impossible, or contrary to the speaker’s expectations. The construction is commonly followed by a negative result, warning, or evaluation.
Usage note
ていては is not itself a polite form; politeness comes from the rest of the sentence. Do not confuse it with てはいけない, which directly means that an action is prohibited.
How to use it
長時間働いていては、健康を損ないます。
If you keep working long hours, you will damage your health.
長時間働いていては、健康を損なう。
If you keep working long hours, you will damage your health.
Example sentences
こんなに電気をつけていては、今月の電気代が高くなってしまいます。
If we keep leaving this many lights on, this month's electricity bill will end up being high.
毎日夜更かししていては、朝の授業に集中できません。
If you keep staying up late every night, you will not be able to concentrate in morning classes.
計画を先延ばしにしていては、締め切りに間に合わなくなります。