Commentary for Kiddushin 67:20

Sukkah and lulav can be performed only on Sukkot, and the shofar only on Rosh Hashanah.
Tzitzit is not worn at night, and tefillin are not worn on Shabbat or holidays. Thus these too have a set time.

Most of these are self-explanatory. The guard rail refers to the commandment to put a railing around one’s roof so that no one falls off.

There are many positive time-bound commandments in which women are obligated and there are positive non-time bound commandments that they are exempt from. So how can the Mishnah propose a general rule?

R. Yohanan says that we simply do not learn from general rules, even if the rule cites a particular exception. He proves this from the case of food used to make an eruv (a common meal that transforms a courtyard into the domain of one person) or a shittuf (the same thing but with an alleyway). The mishnah states that any food product can be used except water or salt because these are not really foods. However, there is another exception—mushrooms, which also are not considered a food, assumedly because they don’t really have any true substance to them.
So what then is the point of a general rule? It seems like these are rules of thumb. They are there because they are generally true, and they help one remember the rule. But they are not strong enough to derive from them any halakhah. Knowing that women are exempt from positive time bound commandments helps one remember what they are obligated in and what they are not. But one cannot derive from here any halakhah.

R. Yohanan says that we simply do not learn from general rules, even if the rule cites a particular exception. He proves this from the case of food used to make an eruv (a common meal that transforms a courtyard into the domain of one person) or a shittuf (the same thing but with an alleyway). The mishnah states that any food product can be used except water or salt because these are not really foods. However, there is another exception—mushrooms, which also are not considered a food, assumedly because they don’t really have any true substance to them.
So what then is the point of a general rule? It seems like these are rules of thumb. They are there because they are generally true, and they help one remember the rule. But they are not strong enough to derive from them any halakhah. Knowing that women are exempt from positive time bound commandments helps one remember what they are obligated in and what they are not. But one cannot derive from here any halakhah.

To find out why women are exempt from time-bound commandments, the Talmud seeks a paradigm and finds it in tefillin. Women are exempt from tefillin, because tefillin is compared to the study of Torah in Deuteronomy 6:7-8 (the first paragraph of Shema). And from Deuteronomy 11:19 we derive that women are exempt from Talmud Torah.

The Shema also mentions the mezuzah, and women are obligated in mezuzah. So why not say that tefillin are compared to mezuzah and that women should be obligated to wear them.
The answer is that tefillin is immediately juxtaposed to the study of Torah in both the first and second paragraphs of the Shema, but it is juxtaposed to mezuzah only in the first. Therefore tefillin is more like Talmud Torah than mezuzah.

If the second chapter of Shema compares mezuzah to Talmud Torah, then maybe women should not be obligated in the mitzvah of mezuzah. So why are they obligated?
The answer is that immediately after the mitzvah of mezuzah, the Torah says “that your days be multiplied”—that you live a long life. Since women want and need life as much as men, they too are obligated in mezuzah.

Sukkah is a positive time-bound commandment. Women are exempt but not because of a general rule. Rather, there is a special midrash on the word “ha’ezrah”—often translated as the citizen—that exempts them. Were this word not written there, it would seem that, contrary to the rule, they would be obligated.

Abaye explains why we need a separate teaching to exempt women from dwelling in the sukkah. There is a midrash on the words “You shall dwell” which are generally interpreted to mean that you shall dwell in the sukkah as you dwell in your house. Since husbands and wives live together in their house, perhaps they should also live together in the sukkah. Therefore, the word “ha-ezrah” comes to teach that women are exempt from the mitzvah of sukkah.

Rava points out another reason we might have thought women were liable for the sukkah—it falls on the fifteenth of the month, as does Pesah. Since women are obligated to eat matzah on Pesah which falls on the fifteenth of the month, so too they would be liable for sukkah which falls on the fifteenth of the month. This is why we need the midrash to teach that they are exempt.