Appendix:Old Irish class B I present verbs

Old Irish class B I verbs (Strachan's A1; McCone's S1a–c) are derived from Proto-Celtic simple thematic verbs ending in, from Proto-Indo-European thematic verbs in. They correspond to the Latin third conjugation and the. The stem-final consonant is always non-palatalized outside of the second and third-person singular and second-person plural. Subclasses differ upon the palatalization of those three forms.

The three subtypes of B I verbs are:
 * S1a verbs are typically exemplified with . They palatalize the second-person singular, third-person singular, and second-person plural in both absolute and conjunct forms, and show alteration of the stem vowel through i- and u-affection. This is the typical kind of B I verb.
 * S1b verbs have a root vowel in e or é and a root-final dental fricative. A typical example is and its compounds. They may behave like S1a verbs when the root is stressed. But when the root is unstressed in a compound, S1b verbs lose their palatalization in the third-person singular and the root fricative becomes a final t . This t often analogically spread to not only the other singular forms, but also formations where the root is stressed, such as what happened to.
 * S1c verbs are exemplified by, and often have a root vowel in o or a. They do not palatalize the stem-final consonant except when it is word-final (in the third-person singular present conjunct and the second-person singular imperative).
 * S1d is McCone's label for Thurneysen's class B III, and are discussed in a separate appendix. They are characterized by a nasal infix in the present stem that is absent in other stems, and can either have S1a or S1c-style palatalization patterns.

There are apparently no deponent verbs in this class;, which comes from and historically ought to be in this class, has shifted to class A II.

B I verbs take either a t-preterite typically when the root ends in a liquid and a suffixless preterite (usually with either root reduplication or the vowel being replaced by á) elsewhere, with only three exceptions,, and  taking the s-preterite.

See Category:Old Irish class B I present verbs for a list of verbs belonging to this class.

Basic pattern
The basic pattern is shown by class S1a verbs and by most class S1b verbs in which the stem vowel is stressed. Two important differences between this class and all other conjugation classes are: (1) that the first-person singular present conjunct form is endingless (with u-coloring of the stem vowel where possible) rather than -u/-im, and (2) that the passive singular present and imperative forms have no -th- before the -a(i)r. A sample verb for class S1a is. The endings are as follows:

Some verbs have different vowels than the basic pattern, but the endings are the same.

One subclass consists of verbs like and, which change historical e to ai when the stem-final consonant is palatalized:

Another subclass consists of verbs like and, in which the stem vowel is i, which is lowered to e when the stem-final consonant is nonpalatalized:

Another subclass consists of verbs in which the stem vowel is éi, which is diphthongized to ía when the stem-final consonant is nonpalatalized. A regular example is, but what is much more widely attested are the regular forms of the irregular verb , which use the stem.

See Appendix:Old Irish delenition for the change of ending-initial ⟨th⟩ to ⟨t⟩ after certain consonants, as seen in S1b compound verbs.

S1b conjugation
This subclass consists of B I verbs whose present stem ends in or  and which end in nonpalatalized  in the third-person singular present indicative conjunct. Some of these verbs, such as and, follow the basic pattern outlined above when the stem vowel is stressed (third-person singular conjunct forms   and  ), but when the stem vowel is unstressed, the final consonant becomes  (spelled ⟨t⟩). This happens when the verb has two (or more) preverbal prefixes, as the stress always falls after the first preverbal prefix. Some examples:
 * and <
 * < (from )
 * and <
 * < (from )
 * and <
 * and <
 * and <

In many cases, this ⟨t⟩ has spread beyond its original location and can be found after a stressed stem vowel or in first- or second-person singular forms:

Synchronically, (conjunct ), whose first-person plural form is attested as, belongs here too, as the first-person plural form shows that the stem-final consonant is. Historically, however, the stem comes from, whose -zd- is expected to give in Old Irish, which means the  of the third-person singular is phonologically regular, and the  of the first-person plural has arisen by analogy with verbs like ones above whose stem historically ends in.

The of the third-person singular conjunct originated in prototonic forms like, where a regular rule of syncope produced , which underwent delenition to.

S1c conjugation
A sample verb of this subclass is. The endings are as follows (note that several of the endings are delenited after the root-final n):