Should not do with 〜てはいけない

Saying one should not perform an action is conveyed by a verb in its て form, the particle は, and the verb いけない (“wrong”, “not good”, “must not do”). Although this does not strictly prohibit an action, it gives a sense of it being wrong to do the action.

See also

To prohibit (rather than simply discourage) an action, 〜てはだめだ is used.
For an old-fashioned way to strongly forbid an action, 〜べからず is used.

Examples

Don't underestimate bread-baking with 〜てはいけない

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Further Reading

Chino, Naoko. "Commands and Prohibitions." Japanese Verbs at a Glance, Kodansha, 2000, pp. 35–36
Chino, Naoko. "Basic Pattern 27." A Dictionary of Basic Japanese Sentence Patterns, Kodansha, 2000, p. 192
Kamiya, Takeo. "Usage of Verb Forms." The Handbook of Japanese Verbs, Kodansha, 2001, p. 160
Kamiya, Taeko. "Commands, Requests, Suggestions, Approval, Disapproval, Prohibition, and Obligation." Japanese Sentence Patterns for Effective Communication, Kodansha, 2005, p. 175
Makino, Seiichi and Michio Tsutsui. "Main Entries." A Dictionary of Basic Japanese Grammar, The Japan Times Ltd., 1989, p. 528