Joe, thank you very much for the answer!
Now it is clear for me that there are five basic forms of verbs (for example: steal, steals, stole, stealing, stolen) and some of them can have different functions.
But I still don’t understand some things.
1. Why do you say that “there are only three word forms that will strictly meet this definition—the word’s base form, its singular form, and its plural form”. After your explanations on verbs I would say that a noun has two forms – the base form (which can also function as a singular form) and its plural form. Why is it wrong?
2. I think that I understood the difference between the base form and a dictionary form, but is it correct to say that “lexeme” is another thing too? That is, we can say that with a verb we have a set of basic forms (including the base form), the dictionary form and the lexeme.
3. I am definitely perplexed with inflections. You say that “has worked,” “have worked” etc. can be regarded as inflections. But it seems that some books think that an inflection is the name only for the extra letter or letters added to a word. Can we say that we have different notions of the word “inflection”?
4. As for the idea that “tenses should be morphological ones only”, I’ve found some grammar books where they try to explain it.
For example, Michael Lewis in his ‘The English verb” writes that ‘…tense is a technical term. It means that there is a morphological change in the base form of the verb. A verb form which is made with an auxiliary is not, in this technical meaning, a “tense”. In this technical sense, the, English verbs have only two tenses…’
Longman grammar: From a structural point of view, English verbs are inflected for only two tenses – present and past.
‘The grammar book’: … the system is selective because tense, in the morphological sense, refers only to the inflections one can use with finite (i.e. inflectionable) verbs. Given this perspective, English has only two tense forms – past and present.
I wouldn’t say that such explanations are clear for me…
P.S. When I was writing ‘That is, it is clear for me that “worker” is a form of the word “work” ‘ I had in mind something like ‘it is clear for me that “houses” is a form of the word “house” ‘, I don’t know why I wrote such a stupid thing instead