A number of English verbs have the same form in the base verb, past tense and past participle. These verbs are all monosyllabic and end in either a t or a d. In the majority of cases, the final consonant is a t.
The English verbs which do not vary in the base verb, past tense and past participle belong to a small group of verbs. These verbs include the following:
bet, bust, bid, cast, cost, cut, fit, hit, hurt, knit, let, read, rid, set,shed, slit, spit, split, spread, wed, wet.
In this list of 21 words, 15 end in t and 6 end in d. However, not all of these verbs are necessarily invariable.
The verb "bust" also has the form "busted" in the past tense and the past participle. Some speakers use "busted" instead of "bust." The verb "knit" also has the form "knitted" in the past tense and past participle. This is used by many speakers. In the case of "spit," the form "spat" is also possible in the past tense and past participle. This is the same pattern as with the verb "sit." The verb "fit" is special because if it is a transitive verb, the form "fitted" is used in the past tense and past participle. It is true that "read" is invariable with respect to orthography, but the base verb is pronounced with a different vowel sound that the past tense and the past participle.
English is a language with a large number of irregular verbs. A few of them share the same form in the base verb, past tense and past participle. However, a few of these verbs are not invariable for all speakers because they have optional forms for the past tense and past participle. They are easy to learn because they they have only three forms: the base verb, the continuous which adds -ing and the third person singular which adds -s.