(Mishnah): If one finds Tefilin [on Shabbos in a field, where they are prone to be disgraced] he brings them in [to the city] one pair at a time;

R. Gamliel says, he brings two pairs at a time.

This refers to old (this will be defined) Tefilin. One who finds new Tefilin is exempt.

If one finds Tzevasim or Krichos (bundles of Tefilin wrapped together), he waits there until dark and brings them [after Shabbos];

If it is dangerous [to wait there, due to bandits], one covers them and goes away.

(Gemara): If one finds Tefilin, he brings them in one pair at a time in the way of wearing them, one on the head and one on the arm;

R. Gamliel says, he brings two [at a time] - two on the head and two on the arm.

(R. Avahu citing R. Elazar): One who puts [on] Tefilin at night transgresses an Aseh - "v'Shamarta Es ha'Chukah ha'Zos l'Mo'adah mi'Yamim Yamimah'' - Yamim, and not nights.

Question: R. Avahu sat and taught in the evening, and Tefilin were on him!

Answer #1: They were to the side (not in the proper place). They were tied to his hand merely to guard them (not l'Shem Mitzvah).

Answer #2: Some say [that R. Avahu said that at night he transgresses] only if he put them on then, but if they were on him during the day, it is permitted [to leave them on].

Some say that the Mitzvah [to wear Tefilin] is until there are no people going in the market.