When introducing the mitzva of tefillin, the Torah says: "You shall tie them on your hands and they shall be a totafos on your head." The Torah does not specify the time or duration of the mitzva. According to some poskim, they are to be worn the entire day (and night, if not for the Chachamim’s ban on wearing tefillin at night – see Shulcha Aruch 30:2). Other poskim hold that the mitzva is to wear them for at least one second sometime during the day, and preferably for the entire day. There is a longstanding custom not to wear the tefillin throughout the entire day out of concern that they would not end up being treated properly.
(סעיף ב וביה"ל ד"ה מצותן)
If one discovers that his tefillin were pasul
According to some poskim, the halacha for someone who wore tefillin based on a chazaka that they were kosher but later learned (after an inspection) that they had never been kosher is similar to the halacha in the case of a kohen who served on the mizbayach before it was discovered that he was disqualified: whatever avoda he had done is kosher. According to this, even the berachos that were made are not considered to have been for naught. Some add that this case falls into the category of someone who wished to perform a mitzva but was prevented from doing so, in which case Hashem considers it as though the mitzva was done. There is an opposite opinion that holds the man in violation of a mitzvas aseh and views his berachos as for naught. (The recitation of Baruch Shem after the second beracha on tefillin mitigates the berachal’vatala.)
(ביאורים ומוספים דרשו, 4)
Reciting Shema before sunrise without tefillin
Chazal considered the recitation of Shema without tefillin as akin to bearing false witness. The Rishonim explain that if a person recites the verses about tefillin when he is not wearing them it is as if he is making a statement that it is unnecessary to fulfill this commandment. However, a person should not wait for tefillin if it is possible that zman kriyas Shema will pass before he has a chance to put them on. Someone who is usually particular to recite Shema before neitz (sunrise) and will miss the opportunity if he waits to don his tefillin may recite Shema without tefillin.
(סעיף ב וס"ק ו; ביאורים ומוספים דרשו, 6)
The main body of the kuf is shaped like a chaf whose base is shortened on the left side. The left leg resembles a mini final nun (with a narrow top and short leg).
The Gemara states, "Why does sheker have only one foot? Because it does not endure." The poskim derive from this that the shin should sit on a [left handed] point and not a flat base.
There is a rule known as mukaf gevil which mandates that every letter of stam must be completely surrounded by parchment. Chazal derived this halacha from the word u’chesavtom, which can be divided into two words – u’chesav and tom-i.e. "complete writing."
The obligation of chinuch with respect to tefillin
May women wear tefillin?
Who must don his tefillin only after reciting Ahavah Rabbah?