SuttaCentral
Sorry if my question is too obvious: in lesson 1 we have, bhavāmi for first person singular and bhavāma for first person plural, why the letter a is lengthened/transformed into ā? On the other hand, we don’t see such lengthening/transforming happens for bhavati, bhavasi, bhavanti, bhavatha.
Also, is there any resources to help me understand more with clear examples for “Vowel gradation”?
The rule is before adding the first person endings mi & ma, the final ‘a’ of an a-ending verb-stem (such as ‘bhava’) is lengthened to ‘ā’.
Since most verb-stems do end in ‘a’, the rule applies to first-person forms of most verbs.
[Background: Pāli inherits the rule from Vedic/Sanskrit, and I think it goes back further into Indo-Iranian (since the same rule exists in Old-Iranic languages which were sister-languages of Sanskrit), and presumably further back into Indo-European (ancient greek too has a similar rule, cf. dadāmi in Pāli & Sanskrit = dídōmi in Greek). In the sanskrit grammatical tradition, this rule is called the ‘ato dīrgho yañi’ rule.]
Thank you. I have learnt something new from you. 
Thank you for your question.
I’ve been too overwhelmed with all these cases and conjugated forms to notice anything.
Now I’ve got another piece of useful info for my notebook. 
Can you suggest a reason for this rule?
This is usually explained as the ending being e.g. -ama. The two a’s smash together to make a long ā
Ah, now that looks familiar.
According to Kaccāyana (which I am reading furiously at the moment, forgive me for being quiet in recent days):
[V] Akāro dīgha’māpajjate hi,mi,ma-icce’tesu vibhattīsu.
When “hi, mi, ma” vibhattis are applied in a verb, the vowel before them is to be made into a dīgha (i.e. lengthened).
[“mhe” vibhatti is also applicable for the function of this Sutta. The applied function is shown in bold, underlined]
• gacchāhi-(You) go. [wa] gamu+a+hi
• gacchāmi-(I) go. [wa] gamu+a+mi
• gacchāma-(We) go. [wa] gamu+a+ma
• gacchāmhe-(We) go. [wa] gamu+a+mhe
Mikāraggahaṇena hivibhattimhi akāro kvaci na dīgha’māpajjate.
By means of “mi” in Sutta, the dīgha procedure is inapplicable in some words (even if “hi” is applied).
It seems that Ven Khemarato and Christie share my feeling that this is a phonetic rule. I was wondering why @srkris expresses it as a grammatical rule that relates to mophology. It seems clear that the reasons are to do with neighbouring sounds, not with which person of the verb it is.
EDITED in response to Dheerayupa’s comment immediately below.
In linguistics, the grammar of a natural language is its set of structural rules on usage and creation of clauses, phrases, and words.
The term can also refer to the study of such rules, a subject that includes phonology, morphology, and syntax, together with phonetics, semantics, and pragmatics.
Sorry, I was trying to keep my language simple. The Oxford English Dictionary (where did you quote from?) says:
Grammar The whole system and structure of a language or of languages in general, usually taken as consisting of syntax and morphology (including inflections) and sometimes also phonology and semantics.
Bhante @sujato is going to start getting cross with us.
Bhante has a sense of humour… (I hope )
I did, then I started teaching Pali. Let’s see if it lasts! 