Tamil (and other Dravidian languages) differ from, e.g., English in this respect, since it seems to be the case that English speakers share the presupposition that an action is completed unless otherwise stated, whereas Tamil speakers seem to share the presupposition that an action is not completed unless stated as definitely complete. Thus a sentence like `I went to the library yesterday' seems odd if followed by `but I never got there.' Rather, the first sentence would have to be replaced by something like `I started out for the library yesterday' if it is to be followed by `but I never got there.' In Tamil, in contrast, the analogous sentence naan neettu laybreerikki pooneen is not strange if followed by aanaa, pooy seeralle (`I never arrived.) since pooneen `I went' is unmarked for completion---it declares simply that motion away from the addressee occurred. If pooneen where changed to add aspectual (v)iDu, i.e., pooyTTeen then adding aanaa, pooy seeralle is odd. (As H&T point out, it tries to "cancel" the implicature of completion, which it cannot do.)
It should also be noted, however, that when the verb of motion is poo which means that motion is away from the speaker/hearer etc., knowledge of whether an event was completed is absent so in most cases in modern ST, pooneen will be considered inadequate (i.e. too vague), and a verb will be marked for completion, i.e. pooyTTeen or pooyirukkeen (use of iru for 'perfect') will be more usual. If the main verb were vaa 'come' there would be no problem, because knowledge of where people are (as a result of coming, i.e. which means toward the speaker/hearer) is available, and verifiable, so with vaa there is often no need for marking completion.
Go to next section: the aspectual verb koo
(LT koL ) and its various avatars.