Balto-Slavic Infinitive and PIE 3rd p., sg, present endings

[bytebuster]

Q: Balto-Slavic Infinitive and PIE 3rd p., sg, present endings

I'm curious to ask if the suffix -tī for the infinitive in Balto-Slavic is related to the PIE third person, singular, present suffix -ti?

Although there is no reason (from a functional point of reasoning) to suppose such a relation, I recently learnt that the only "dialect" of Balto-Slavic that has undergone a transition to analyticity, namely Bulgaro-Macedonian, has lost both the infinitive and the ending -ti in 3rd p, sg verb conjugation, so morphologically there is some point in conjecturing some relation...

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Моя відповідь:

TL;DR: Infinitive suffix "-ти" and 3rd person singular present "-тъ" are essentially different.
They could reduce independently in Slavonic languages, but if they are both reduced, this seems to be just a coincidence.

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In his "Slavonic Grammar with Correct Syntax" (1619), Meletius Smotrytsky marks 3rd person singular present suffix -тъ (pronounced with short [ɔ]).

So, the verb to read, "читати" [t͡ɕɨ-ta-ti] conjugates to "чтєтъ" [t͡ɕtɛ-tɔ].

Here's the scan of Smotrytsky's work

Links: exact page, title page.

In modern Slavonic languages, indeed, these suffixed often reduce.

Ukrainian (my native language): the infinitive suffix remains the same, while the 3rd person singular present suffix reduces: "читати" [t͡ɕɨ-ta-tɨ] → "читає" [t͡ɕɨ-ta-ʲe] or even "чита" [t͡ɕɨ-ta] in Western dialects.

On the contrary, the modern Russian reduced suffix in the infinitive form while it retains 3rd person singular present suffix: "читать" [t͡ɕi-tatʲ] → "читает" [t͡ɕi-ta-ʲet].

So, as we can see, the distinction between Ukrainian and Russian suggests that both suffixes may retract independently.