Commento su Miqwa'ot 2:1
הַטָּמֵא שֶׁיָּרַד לִטְבֹּל, סָפֵק טָבַל סָפֵק לֹא טָבַל, אֲפִלּוּ טָבַל, סָפֵק יֶשׁ בּוֹ אַרְבָּעִים סְאָה סָפֵק אֵין בּוֹ. שְׁנֵי מִקְוָאוֹת, אֶחָד יֶשׁ בּוֹ אַרְבָּעִים סְאָה וְאֶחָד שֶׁאֵין בּוֹ, טָבַל בְּאַחַד מֵהֶם וְאֵינוֹ יוֹדֵעַ בְּאֵיזֶה מֵהֶן טָבַל, סְפֵקוֹ טָמֵא:
Qualcuno che è sceso per immergersi [in un mikveh ], e c'è un'incertezza sul fatto che si sia immerso o no, o un'incertezza sul fatto che contenesse quaranta se'ah [una specifica unità di volume] in esso o no ; o se ci fossero due mikva'ot [raduni d'acqua raccolti con mezzi naturali e immersi per la purificazione], uno con quaranta se'ah in esso, e uno senza, e qualcuno immerso in uno di essi ma non sa in quale uno che ha immerso, la sua incertezza è impura [cioè il caso è incerto e risolto come impuro].
Bartenura on Mishnah Mikvaot
English Explanation of Mishnah Mikvaot
If it is doubtful whether he did immerse himself or not;
And even if he did immerse himself, it is doubtful whether the mikveh contained forty seahs or not;
And if there were two mikvehs, one containing forty seahs but the other not containing forty seahs, and he immersed himself in one of them but he does not know in which of them he immersed himself,
In such a doubt he is unclean.
Today's mishnah deals with a case of an impure person who immersed himself in a mikveh in order to become pure, but it is unclear whether he did so in a manner that would purify him. In all such cases he is not considered clean. The reason is that his last determinable status was unclean. Therefore, he remains unclean until we can be sure that he is actually clean.
The mishnah is pretty straightforward, so there is no explanation below.