Possibly the difference is whether or not the Torah had a special reason for saying the words "U'lekachtem" or "U'keshartem" specifically in the plural form. The Torah did not have to say "U'lekachtem" in the plural form but could have used the singular "U'lekachta". The fact that the plural form was used means that there is something extra which can be used to learn a new Halachah. This is what RASHI SUKAH 34b DH TAMAH writes that the Derashah is from "U'lekachta Tam" i.e. the word "Tam" is superfluous and teaches that the taking of the Lulav must be complete i.e. all 4 species.

In contrast there is a special reason why the second paragraph of the Shema is written in the plural form. This teaches that what the Torah commands in the Shema is both a Mitzvah on every individual and a Mitzvah on the community (see RASHI at end of Devarim 11:13). Therefore one cannot make a Derashah from the word "Tam" in "U'keshartem" because there is a special reason why the Torah could not have written "U'kesharta".

KOL TUV

D. Bloom

It may be possible to suggest another difference between "u'Lkachtem" and between "u'Kshartem" based on the commentary of MALBIM to Vayikra 23:40.

There Malbim explains that there is a difference between 2 types of future forms in Hebrew:

(1) A word which is made future by a "Vav Mehapech" - a "Vav" which converts the verb from the past to the future tense. For instance, u'Lkachtem , where "Lekachtem" is past tense, but is made future by the Vav.

(2) A word which is totally future i.e. a normal future tense which does not require a Vav to convert it into future.

Malbim writes that the activity represented by (1) is completed by one action whilst (2) represents a continuous activity.

An example of (2) is Bereishis 27:28 where Yitzchak blessed Ya'akov "And G-d will give you from the dew of Heaven and the fats of the earth". RASHI explains there that "will give you" means that He will give and afterwards also continue to give to you. Because "will give you " is an absolute future, without any "Converting Vav", this means the giving will be continuous. In contrast u'Lkachtem, which is only made future by the Vav, means a one-time taking. Therefore the taking of the Lulav must be done in one shot, whilst if one of the 4 species is not available, the Mitzvah is invalid.

However Malbim adds that there is an additional reason why u'Lkachtem signifies that all 4 species must be taken. In the previous verse, 39, the Torah says "Tachogu" in the future tense - You shall celebrate the Festival of Sukos. The fact that the Torah switched from the future of Tachogu to the converted past of u'Lkachtem in the next verse (instead of saying "u'Kechu" - "and take" - which would have indicated continuous taking) proves that the Torah was particular that u'Lkachtem should not be a continuous activity. In contrast in Devarim 11:18, when the Torah states "u'Kshartem", there is no switch from the total future tense to the converted future, so one cannot infer that u'Kshartem does not mean continuous tying.

Therefore even if the hand-Tefilin was worn without the head-Tefilin, or vice versa, one still thereby performed a Mitzvah of Tefilin.

(See also TORAH TEMIMAH Vayikra 23:40 #120, who agrees with Malbim).

KOL TUV

D. Bloom

Here is another answer to your question. The Gemara in Menachos 35b does actually derive from the verse "u'Keshartem" that one needs "Keshirah Tamah", "a perfect tying", but the Gemara gives a different sort of explanation of what Keshirah Tamah is. In contrast to "Lekichah Tamah", which teaches that one must take all four species together, Keshirah Tamah teaches, according to the first explanation of RASHI DH KESHIRAH, that there must not be an additional knot on the strap, apart from the knot with which one ties the Tefilin on the head because otherwise this will not be a beautiful Keshirah, which is the meaning of Keshirah Tamah. According to the second Pshat of Rashi the reason that the Tefilin are invalid if there is an extra knot on the strap, is because Keshirah Tamah teaches that the strap must be complete, without any knots on it.

It may be that the reason Menachos 35b does not learn Keshirah Tamah in a similar way to Lekichah Tamah, which would have taught that one must have both Head and Arm Tefilin, just like one must have all 4 species, is because the Gemara prefers to derive from the words Keshirah Tamah a Din which is directly related to the tying - Keshirah - of the Tefilin. Therefore it learns that if there is an extra knot on the strap, the Tefilin is invalid. In contrast if there would have been a Din that one must have both Head and Arm Tefilin,this Din would not have related specifically to the knots of the Tefilin.

Therefore since Menachos 35b used "u'Lekachtem" to teach the Din that there must be no extra knots on the Tefilin strap, there was no verse left over from which one could derive that if one did not have both Head and Arm Tefilin, the entire Mitzvah is invalid.

YEYASHER KOACH

D. Bloom

If the Torah would have wanted to say "You (singular) shall take them (i.e. the plural 4 species) it could have said Ul'kachtom with a "Kometz" under the "Tof". The fact that the Torah actually said Ul'kachtem with a "Segol" under the "Tof" means that there is something superfluous which we can use for the Drashah of Lekichah Tamah.

This would be similar to the difference between Devarim 6:8 where a "Kometz" is underneath the "Tof" and between Devarim 11:18 where a "Segol" is underneath the "Tof". Devarim 6:8 is "You (singular) shall tie them (i.e. the words of the Shema)" whilst Devarim 11:18 is plural and commands the entire People to tie the words upon them .

I suggested in my first reply that since Vayikra 23:40 used the plural with the Segol, not the singular with the Kometz, this means there is something extra which can be used to teach that if all 4 species are not present, the Mitzvah is invalid.

KOL TUV

D. Bloom