The verb téigh completely changes it's form in the past tense when forming a question or a negative proposition, so:

chuaigh sé = he went
an ndeachaigh sé? = did he go?
ní dheachaigh sé = he did not go

Effectively, a 'dea' stem is added to the verb and the 'u' is removed after 'ch'. It doesn't do this in any of the other tenses as far as I can see. And I haven't come across another verb that does this.

Is there an explanation for this? I am perfectly willing to accept the answer: 'It just is what it is'.

Some irregular verbs have different stems in the absolute (chuaigh) and dependent (ndeachaigh) forms. The same is seem with: táim, níl. Níl is short for ní fhuil, and so tá- is absolute and fuil- is dependent. You could compare it to the way the verb "to go" in English has conjugated forms like "went", and "to be" has forms like "is" and "was", showing at least 3 different original roots. Actually, in Cork Irish, you can say "níor chuaigh sé", so some of the dialects have different takes on this.

That is very helpful. Thank you for the quick response.

Many different verb forms in irregular verbs are due to Old Irish stress patterns and resulting changes.
Some verb forms (following most verbal particles, conjunctions) had prototonic stress (on the first syllable of a compound verb, the first syllable was usually a prefix or so-called "preverb") while other forms had deuterotonic stress (on the second syllable of the same verb, either on the main syllable of the verb, its root, or on a second preverb)
Unstressed syllables became weak and reduced or they disappeared totally, stressed syllables changed their vowels, etc.

Deuterotonic chuaigh, old spelling chuaidh (< dochoad) and prototonic deachaigh , old spelling deachaidh (< dechaid) are among them.

I came to say more-or-less this. Modern irregular verbs are often the result of old linguistic patterns not being removed from the language's most commonly used words as grammatical patterns change. It's typically the case that the verbs which are used most frequently in a language resist change the longest. Hence, we tell children that "taked", "putted" and "readed" are not the correct past forms for "take", "put" and "read" in English, even though they match the standard grammatical pattern for past tense verbs.

As it turns out, however, this is only half of the story for Irish téigh. This verb was already highly irregular even in Old Irish. To demonstrate just four roots it already had by the 8th century, see the following examples from different tenses:

Present - tíagu "I go", téigi "thou goest", téit "he/she goes", tíagmai "we go", téit "you go", tíagait "they go".
Preterite - lod "I went", lod "thou went", luid "he/she went", lodmar "we went", *lodaid "you went", lotar "they went".
Past perfect - dochood "I went", dochuad "thou went", dochoid "he/she went", dochuamar "we went", dochuabair "you went", docotar "they went".
Future - raga "I will go", ragai "thou wilt go", ragaid "he/she will go", regma "we will go", ragthai "you will go", ragait "they will go".

(*lodaid is unattested, but is possible as it would follow the expected pattern of the suffixless preterite in Old Irish.)



As djwebb says, it's not unusual for irregular verbs to be comprised of different stems/roots in different tenses. The English example he uses portrays this well, the past tense of Modern English "go" was adopted originally from an entirely different verb. Early Modern English still had the verb "wend" (from Old English gān), the past tense of which was "went". This was simply used more often in the past tense, while "go" was used more often in the present. When "wend" ceased to be used in the present, "went" continued in the past tense. This same phenomenon is responsible for the distinction between the different roots of the Old Irish verb téit which I just gave above. Moreover, this Old Irish distinction is continued in the difference between Modern Irish téigh (from the Old Irish root téit) and chuaigh (from the Old Irish root dochoid).

This is to say nothing about the distinction between absolute and dependent, however. In the case of téigh, it is only the different tenses which were drawn originally from distinct roots. The distinction the between absolute and dependent forms has more to do with stress patterns in the language, as Labhrás mentioned:



You can actually see that the forms which eventually became chuaigh and -deachaigh were originally much more similar to each other in Old Irish. Compare the past perfect absolute forms I gave above to these Old Irish dependent forms:

Past perfect (dependent) - ni-dechud "I didn't go", (ní)*-dechud "thou (didst not) go", (ní)-dechuid "he/she (didn't) go", -dechummar "we (didn't) go", (ní)*-dechabar "you (didn't) go", (ní)-dechaddar "they (didn't) go".

(*-dechud and *-dechabar are unattested, but seem likely based on their absolute forms.)

By the late Middle Irish period the preverb, do-, spread to the past tense of many verbs, but only to the absolute forms, not the dependent forms. Old Irish dochoid already had this preverb from a much earlier stage, however, and therefore its dependent form, (ní)-dechuid, had the preverb also. It's that initial -de. Modern Irish (for the most part) has now dropped this preverbal do- from the absolute forms of past tense verbs, hence, Old Irish dochoid became just chuaidh. As the dependent forms of verbs never took the Middle Irish preverb do- to begin with, however, there was nothing to drop from them in most cases. The exception is the dependent form, -deachaigh, which had retained the preverb from Old Irish (ní)-dechuid. This is why the absolute and conjunct forms of this verb look so different in Modern Irish.

Thank you all for those very comprehensive explanations.