1. Middle English period great change Middle English Verbal System

Anomalous verbs

Also known as "irregular verbs", these verbs play important grammatical roles (for instance, all of them function as either modal verbs or auxiliary verbs) and do not easily fit into one of the other conjugational classes. All of them except for don and willen are suppletive, meaning that they historically come from a conflation of multiple distinct verbs. willen is not easily distinguishable from a preterite-present in Middle English, but is placed in this class as it has a different historical origin.
In Middle English, there are just four anomalous verbs: been, don, gon and willen.

Conjugation

Middle English saw little change from Old English with respect to verb conjugations; it continued to display a system typical of the Germanic languages. Some of the major differences are the merger of vowels in unstressed syllables, the levelling of many ablaut alternants, and the gradual weakening of final -(e)n to -(e) in the infinitive, plural, and past participle. Verbs inflect for the following parameters:

• 
  Two tenses: present and past (the future is usually denoted with auxiliaries; sometimes the plain present is used).
  
• 
  Three moods: indicative (the default), subjunctive (denotes hypothetical or counterfactual situations) and imperative (denotes commands; there is no past imperative).
  
• 
  Person and number, agreeing with the subject of the verb phrase.
  

Additionally, there is an infinitive, a present participle, and a past participle (see below).
However, there is a high degree of syncretism; not all possible combinations of parameters have a unique form. In particular, plurals, subjunctives, or imperatives are unmarked for grammatical person, and with a few exceptions, first-person singulars and plurals lack a distinction between the indicative and subjunctive. These rules do not apply for all dialects; but the general trend towards syncretism remains valid.
It should also be noted that the plural form, in conjunction with ye/you, can be used as a formal second-person singular, especially in Late Middle English. Specifically, it is used when one wishes to display politeness and respect, including towards superiors and other individuals which one is societally obligated to show deference to.
The paradigm of the strong verb singen, singe is shown below.

infinitive	(to) singen, singe
	present tense	past tense
1st-person singular	singe	sang
2nd-person singular	singest	sunge, sange
3rd-person singular	singeth	sang
subjunctive singular	singe	sunge, sange
imperative singular		—
plural	singen, singe	sungen, sunge, sangen, sange
imperative plural	singeth, singe	—
participles	singynge	sungen, sunge

While a few weak verbs have become strong, the general tendency is for lesser-used strong verbs to become weak. Similarly, the tendency is to borrow verbs as weak; the only borrowed strong verbs are those which bear a strong resemblance to extant strong verbs (e.g. striven) or which are strong in the (Germanic) source language (e.g. taken). This means strong verbs tend to form a more archaic layer of vocabulary than their weak counterparts.

Southern Middle English retains the -aþ present plural ending of Old English as -eþ, and tends to retain the y- prefix of the past participle to a greater degree. The -ian ending of Old English class 2 weak verbs is retained as -ie and extended freely to loanwords. However, because y- serves as a marker of the past participle, the final -n of past participles is often lost. The usual present-participle ending is -ende, but -ynge penetrates the South later, as it did the Midlands.

In Northern Middle English, the influence of Old Norse is easily discernible. For instance, the infinitive alternatively employs the use of the particle at (e.g. "at sing") similar to Old Norse (cf. að syngva). Additionally, the tendency to maintain a strong final -en of the past participle (cf. Old Norse -inn), use of the present participle -and (cf. Old Norse -andi), and the second-/third-person present indicative ending -es can all be traced to Scandinavian origin.
Unlike in the past participle, the final -n of plurals and infinitives was lost at a relatively early stage (cf. the lack of -n in the equivalent Old Norse forms); -e was then elided as well, meaning that the past plural, the indicative, the (both present and past) first-person singular, and the subjunctive have a zero ending. The usual Northern present plural is not the -en/-e of the Midlands or the -eth of the south, though; instead, -es is often extended from the second-/third-person singular. This -es plural ending is not present in the subjunctive or the past as -es is absent from the corresponding singular forms.
Another prominent characteristic of the Northern verbal system is the levelling of irregular forms, no doubt due to the spread of -es and loss of final -en and -e resulting in there being less distinct inflectional categories to spread over. For instance, can, levelled from the present, is found instead of the infinitive cunnen, and (I/Ik) is sometimes replaces (I/Ich) am.