I would like to add a thought to this issue. For me, the main question is: How does the verb system of a certain language solve the problem of comunicating the intended information?
On the basis of the classification developed by Latin grammarians, verbs provide us with 5 pieces of information.
- Person (1st, 2nd, 3rd)
- Numerus (i.e. singular or plural)
- Tense (present , present progrssive, past perfect, futur perfect, for example)
- Genus verbi (active voice or passive voice)
- Modus (Indikativ, Konjunktiv, Imperativ)
Example: "es wird geputzt worden sein" = "it will have been cleaned" =
- 3rd person
- futur II (future perfect)
- passive voice
- indicative mode
Now, when we would like to modify a certain "main verb" (e.g. "clean"/"putzen") with a modal verb ("can"/"koennen", "must"/"muessen), this classification leads to the question of whether the 5 pieces of information mentioned above are provided by means of inflecting the modal verb or the main verb (or a combination thereof).
English and German HAVE FOUND DIFFERENT ANSWERS to this question.
Let's first have a look at how the English language addresses this problem.
1st person sg futurII active indicative, main verb: read, modifier: can/be able to
- "I will have been able to read."
3rd person sg futurII passive indicative, main verb: clean, modifier: must/have to
- "It will have had to be cleaned."
In the English case, the modal verbs "can/be able to" and "must/have to" carry the information about person, number, tense and mode.
The main verbs "read"/"clean" are added as infinitives ("read"/"be cleaned") and provide us with the information about the Genus verbi (active/passive voice).
1st person sg futurII active indicative, main verb: lesen, modifier: koennen
- "Ich werde gelesen haben koennen."
3rd person sg futurII passive indicative, main verb: putzen, modifier: muessen
- "Es wird geputzt worden sein muessen."
In the German case, it is the main verbs that carry ALL the information about person, number, tense, active/passive voice and mode.
The modal verbs are always added as plain infinitives (active voice).
In sum: My point is that we can avoid a lot of confusion if we take into account that English and German use different strategies to convey the 5 pieces of essential information to the reader, namely by distributing the information between the main verb and the modal verb in a different way.