16th century, equivalent to be- + ja (“yes”) + -en (with -h- as orthographic hiatus breaker). While the verb may have been derived directly from the interjection ja, an alternative possibility is that it is a reinterpretation of Middle High German bejëhen (“to acknowledge, admit”), from jëhen (“to say”). The according spelling bejähen is indeed attested in the 17th century.
bejahen (weak, third-person singular present bejaht, past tense bejahte, past participle bejaht, auxiliary haben)
| infinitive | bejahen | ||||
|---|---|---|---|---|---|
| present participle | bejahend | ||||
| past participle | bejaht | ||||
| auxiliary | haben | ||||
| indicative | subjunctive | ||||
| singular | plural | singular | plural | ||
| present | ich bejahe | wir bejahen | i | ich bejahe | wir bejahen |
| du bejahst | ihr bejaht | du bejahest | ihr bejahet | ||
| er bejaht | sie bejahen | er bejahe | sie bejahen | ||
| preterite | ich bejahte | wir bejahten | ii | ich bejahte1 | wir bejahten1 |
| du bejahtest | ihr bejahtet | du bejahtest1 | ihr bejahtet1 | ||
| er bejahte | sie bejahten | er bejahte1 | sie bejahten1 | ||
| imperative | bejah (du) bejahe (du) |
bejaht (ihr) | |||
1Rare except in very formal contexts; alternative in würde normally preferred.