いかんだ / いかんでは / いかんによっては

いかん follows a noun, directly or with の, and means "depending on": the outcome turns on that noun. The noun is something that can go more than one way – a result, someone's effort, a reply, the weather.

A verb can't take いかん directly. Use the noun for the action instead.

• × 努力するいかんだ → 努力いかんだ

いかんだ or いかんでは

いかんだ ends the sentence. What hangs on the factor comes first, marked with は, and いかんだ names the factor.

いかんでは and いかんによっては sit mid-sentence and mean the same as each other. The は picks out one way the factor might go, so the second half is one possible outcome: こともある, かのう可能せい性がある, かねない, or something that would then have to be done.

Without the は, いかんで and いかんによって state a plain dependency: as the factor changes, so does the outcome.

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いかん belongs to formal writing: official notices, contracts, the news, reports. In speech and in ordinary writing, しだい次第 does the same job, and によって or によっては covers the mid-sentence use.

The opposite, "regardless of", is いかんにかかわらず or いかんをとわ問わず. The いかん of western Japanese dialects, meaning "no good" or "mustn't" (いけない), is a different word.