What one needs to know about Tamil syntax to understand the evolution of aspectual verbs:
S / \ / \ NP VP / \ / \ S Vb / \ / \ NP V (AVP)
In other words, all embedding is to the left of the main verb; if Sentences are conjoined or embedded, the verb of the embedded sentence is truncated, i.e. does not appear fully-formed, but appears instead as the AVP (adverbial participle), which is the past stem of the verb with a euphonic vowel (if necessary). There can be multiple embeddings, theoretically infinitely, with only the last of the sentence marked for tense, PNG, modality, etc. (negation is usually marked on the final verb, too, but may also appear on the AVP, i.e. there are negative AVP's. In older Tamil, negation was clearly a verbal element i.e. negation is a verb.)
I have recordings of folk tales where the whole story was one long sentence, each verb of the sentence in the form of the AVP, or perhaps embedded before the AVP of the quotative verb, so that only the last verb of the story was finite.
Here's an example:
|tiDiir-NNu kaakkaa||parandu||vandu||oru muttu maaleye||tuukki||kiTTu||parandu||pooy||iDu-cci|
|Suddenly (the) crow||flying||coming||(the) pearl necklace||lifting||(while) holding||flying||going||quit-PNG|
Aspectual verbs are thus like the "vector verbs" H&T cite from Peter Hook's work on Hindi. Typically they follow main verbs (to the right) but they bear the morphology, while the lexical verb is in the form of the AVP. This is different from auxiliary (modal) verbs, that express ability, permission, possibility etc. which follow a lexical verb in the infinitive form. Modal verbs have more restricted morphology in Tamil, i.e. tend not to be marked as fully for tense, PNG, etc. Modal verbs have also been recruited from lexical verbs, but their syntax is different.
Look here for more information on syntax of modals. For more information on Tamil verb morphology and syntax, consult this source Proceed to more indepth discussion of Tamil aspectual verbs.
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