[This is part of a small but growing series of defectivity case studies.]

English lexical verbs can have up to 5 distinct forms, and I am aware of just a few English verbs which are defective. (The following are all my personal judgments.)

1. I can use begone as an imperative, though it has the form of a past participle (cf. gone and forgone). Is BEGO even a verb lexeme anymore?
2. Fodor (1972), following Lakoff (1970 [1965]), notes that BEWARE has a limited distribution and never bears explicit inflection. For me, it can occur only as a positive imperative (e.g., beware the dog!), with or without emphatic do. I agree with Fodor that it is also bad under negation, but perhaps for unrelated reasons: e.g., *don’t beware… 
3. FORGO lacks a simple past: forgo, forgoes, and forgoing are fine, as is the past participle forgone, but *forwent is bad as the preterite/simple past, and *forgoed is perhaps a bit worse.
4. METHINK can only be used in the 3sg. present active indicative form methinks, and doesn’t allow for an explicit subject.
5. STRIDE lacks a past participle (e.g., Hill 1976:668, Pinker 1999:136f., Pullum and Wilson 1977:770): *stridden is bad.  The simple past strode cannot be reused here, and I cannot use the regular *strided (under the relevant sense).

References

Fodor, J. D. 1972. Beware. Linguistic Inquiry 3: 528-534.
Hill, A. A. 1976. [Obituary:] Albert Henry Marckwardt. Language 52: 667-681.
Lakoff, G. 1970. Irregularity in Syntax. Holt, Rinehart and Winston.
Pinker, S. 1999. Words and Rules: The Ingredients of Language. Basic Books.
Pullum, G. K. and Wilson, D. 1977. Autonomous syntax and the analysis of auxiliaries. Language 53:741-788.