Quenya verbs can be divided up into three broad groups:
- Basic verbs derived directly from some primitive verb stem.
- Derived verbs created by adding a verbal suffix to another root (verbal or otherwise).
- The u-stem or a-stem verbs which have a vocalic addition (a or u) to the stem.
Within these three groups there are more specific verb classes.
The majority of basic verbs are derived from biconsonantal roots of the form √KAT. These are “strong” verbs, and retain the purest expression of Primitive Elvish inflections. A few verbs are derived from triconsonantal √TALAT roots, but phonetic modifications like the Quenya syncope distorted their original form, so that these “talat-stem” verbs have distinct inflections. I tend to reserve the term “basic verb” only for those from biconsonantal verbal roots, using the term “talat-stem” for the verbs from triconsonantal roots, which mirrors Tolkien’s own terminology. Tolkien sometimes used the term “strong verbs” for biconsonantal √KAT-stem basic verbs, but he also used the term “strong” when talking about specific conjugations within other verb classes. The resulting terminology can be rather confusing, which is why I stick to the term “basic verbs” for √KAT-stem verbs.
Derived verbs are the result of various verbal suffixes, the two largest groups being the causative suffixes -tā, -yā and the formative suffixes -t(a), -y(a). The different verbal suffixes interact with inflectional suffixes in various ways. Some derived verbs treat the verbal suffix as invariant, so the inflections are (mostly) added to the verb stem, resulting in the so-called “weak” verbs. Other derived verbs modify the verb stem in various situations, making up the “half-strong” verbs.
Finally, there are the verbs with vocalic additions: the a-stem and u-stem verbs. Some Neo-Quenya writers use the term “a-stem” for any verb stem ending in a, including derived verbs, but Tolkien himself applied the term a-stem only in the limited case of verbs with an a-suffix. In this lexicon, I use more specific terms such as derived verbs and weak verbs for other verbs ending in a.
Conceptual Development: Tolkien’s earliest writings on verbs focused more on the basic conjugations than verb classes: The Qenya Verb Form from the 1910s and Early Qenya Grammar from the 1920s only describe conjugations for basic verbs. Quendian & Common Eldarin Verbal Structure from the 1930s briefly mentions the “Formation of other Verbal Stems” (PE22/98), describing all of the classes mentioned above (TALAT, derived, a-stem, u-stem) without specifically naming them.
The most complete description of verb classes appears in the Quenya Verbal System from the 1940s. In addition to basic verbs, he described five verb classes (PE22/113-114):
- The “half-strong” verbs , further broken up into derived verbs with -t(a)/-y(a) suffixes and talat-stem verbs.
- The a-stem  and u-stem verbs .
- Two categories of derived verbs: verbal stems with causative suffixes -tā/-yā  and non-verbal stems with various suffixes .
In this document, Tolkien used the term “weak” to refer to all of the non-basic verb classes, but I generally limited this term only to the last two groups whose conjugations are very similar. I use the term “half-strong” only for verbs with the -t(a)/-y(a) formative suffixes, using “talat-stem” for verbs from triconsonantal roots.
In Common Eldarin: Verb Structure from the 1950s, Tolkien described Primitive Elvish verb classes using similar terminology. This includes basic verbs versus derived verbs (PE22/129), talat-stem (PE22/133), a-stem and u-stem (PE22/134). Many of these verb classes are also mentioned in Tolkien’s Late Notes on Verb Structure from the late 1960s, including a-stem (PE22/151, 163) and the two main “flavors” of derived verbs, causatives and formatives (PE22/156, 159). Thus it seems Tolkien’s conception of Quenya verb classes remained more or less stable in the 1930s through 1960s, thought the details of their conjugations changed over time.
Neo-Quenya: As can be seen in the references above, considerable new information on verbs was released in Parma Eldalamberon #22, first published in 2015. Prior to that point, most Neo-Quenya writers grouped verbs into only three categories: basic, derived (aka. “a-stem”) and u-stem. The derived verbs were generally assumed to have the weak conjugation. The existence of aberrant behavior for talat-stems and some ya-verbs was known from examples in PE17, but these were not very well understood.
With the release of PE22, the use of the term “a-stem” for all derived verbs became problematic, because we learned that Tolkien himself used this term in a more specific way, for verbs with the vocalic extension a. As such, I recommend using a-stem only for this more limited class, and the more general term “derived verbs” for the larger group.
This “old school” Neo-Quenya classification of verbs into basic, derived and u-stem verbs is not entirely inappropriate, however. Within the larger class of derived and a-stem verbs, the “weak” conjugation is both the largest and most regular. There was a trend for Quenya verbs in similar verb classes to regularize their conjugations to match weak verb inflections. Thus using the weak conjugations “incorrectly” for other derived verbs was probably common within the Legendarium itself, especially among the less learned. In many cases we don’t know exactly which class a verb falls into, and using the weak conjugation as the default for all verb stems ending in a is not a bad approach.