Question: Rav says that Ma'ariv is Reshus, but in Berachos (26a), [he] said that if one erred and did not pray Ma'ariv, he prays Shacharis twice.

Note: In our text, R. Yochanan said so in Berachos. It seems that Tosfos' text there said "Rav". If not, what is his question? (Indeed, Tosfos there questioned R. Yochanan's Halachah, for it is unlike Rav and the Halachah follows Rav. Perhaps he asks why the Gemara did not mention that the Halachah does not follow R. Yochanan. In any case, our Tosfos saw no need to explain what is difficult!) Also the following words of Tosfos cite Berachos 30b, where Rav connotes that Ma'ariv is Chovah. He does not stress that there, Rav himself said so! Also, why didn't Tosfos first ask the more direct question?

[Rav himself] said (Berachos 30b) that if one forgot to mention Rosh Chodesh in Ma'ariv, he need not repeat [Shemoneh Esre], since we are not Mekadesh the month at night.

Inference: If not for this (i.e. if we were Mekadesh at night), he would repeat [Shemoneh Esre].

Summation of question: What is the reason? "Ma'ariv is Reshus" should suffice [to exempt from repeating it]!

Answer (Ri): Even though it is Reshus, one should not be Mevatel it without reason, only for a Mitzvah that cannot be done later, or if he already untied his belt (Shabbos 9b). We do not burden him [to gird himself again to pray];

Similarly, if he already alit onto his bed, the Yerushalmi says that we do not exert him to descend.

Support #1: Since Yakov Avinu enacted Ma'ariv, he did not enact it for naught.

Support #2: It was enacted corresponding to limbs and Chelev. One may put them on the Mizbe'ach and burn them the entire night;

Even though they are not Me'akev Kaparah, the Mitzvah is to burn them;

Similarly, Ma'ariv is a Mitzvah.

Question: What was the Gemara's question here, that Rav holds that Ma'ariv is Reshus? We say that one should not be Mevatel it needlessly!

Answer: This is not needlessly. It is a Mitzvah on the night of [Motza'ei] Yom Kipur to toil to prepare needs of the Seudah, which is like a Seudas Yom Tov;

A Midrash says that on Motza'ei Yom Kipur, a Bas Kol says "go eat your bread with Simchah!"

Support (for Ri): We explained that even according to the opinion that Ma'ariv is Reshus one should not be Mevatel it needlessly. The Yerushalmi connotes like this;

Citation (Yerushalmi): According to the opinion that Ma'ariv is Chovah, Tefilas Ne'ilah does not exempt it. According to the opinion that Ma'ariv is Reshus, Tefilas Ne'ilah exempts it;

Inference: One may not be Mevatel it needlessly.

Implied question: [This is not a support, for] the Yerushalmi argues with the Bavli about this!

The Bavli connotes that Rav taught according to the opinion that Ma'ariv is Chovah, but according to the opinion that Ma'ariv is Reshus, it does not need another Tefilah to exempt it!

Answer: We should equate the Bavli and Yerushalmi as much as possible. (Presumably, the Bavli agrees that all agree that one should not be Mevatel Ma'ariv needlessly.)

Bahag's opinion: Bahag explained regarding "[one who forgot to mention Rosh Chodesh in Ma'ariv does not repeat it, because] we are not Mekadesh the month at night", even though Ma'ariv is Reshus, since he accepted it like a Chovah upon himself, this is fine (it is like a Chovah);

Note: Tosfos ha'Rosh explains that through praying Ma'ariv [tonight], he accepted it like a Chovah upon himself.

Therefore, we need the reason that we are not Mekadesh at night [to exempt him].

Rebuttal (Ri): We say in Shabbos (9b), according to the opinion that Ma'ariv is Reshus, if he already untied his belt, we do not burden him;

Inference: If he did not yet untie his belt, we burden him, even though he did not pray at all [to show that he accepts it like a Chovah]!

Suggestion: Since he prayed on other nights, he accepted it like a Chovah!

Rejection: If so, why did we need to say that Rav taught according to the opinion that Ma'ariv is Chovah?

We should say that even according to himself, who holds that it is Reshus, since he prayed on other nights, he accepted it like a Chovah, so it needs a Tefilah to exempt it!

Conclusion: Rather, it is like I explained.

Explanation (Rashbam): Since he is still fasting, it is considered like Yom Kipur, therefore he prays [only] seven [Berachos];

Since the day already ended, and it is a Yom Chol, therefore he prays like 18 Berachos.