All Hebrew verbs have two forms of imperative. We find 'You shall speak to all the wise of heart' (Exod. 28:3) which is a command. Even though from the point of view of grammar tedaber ('you shall speak') is future tense the connotation is imperative. However the grammatical imperative for the root Dalet Bet Resh is daber as in 'And you speak to the children of Israel' (Exod. 31:13). Generally the grammatical imperative is in the form of the future second person of the same verb, without the Tav prefix of the second person. As the imperative is of its nature second person, the Tav is unnecessary. In addition the shortened form helps to create a sense of urgency that is a nuance of the grammatical imperative. Today, in Hebrew speech, commands are more often than not given in the second person future (including the prefix Tav). The use of the grammatical imperative is of low incidence, having as it does, a nuance of urgency and perhaps also pressure on the person commanded. (Perhaps this is because our generation is so well-mannered!)

Verbs of the Lamed Heh (or Yud) conjugation (ie which have the letter Heh as the third letter of the root) have a third form of imperative - the shortened imperative. Thus for example in the root Nun Kaf Heh, ('to smite') the future second person singular masculine form is takeh (Deut. 13:16), while the grammatical imperative is hakeh (Ezek. 6:11). However we also find vehach (Exod. 8:12). This is the shortened imperative, it omits the third letter of the root, and the word becomes one syllable shorter. All these three forms can be found for the root Tzade Vav Heh. Thus, 'and you shall command the children of Israel,'(Exod. 27:20) uses tetzaveh - this is the future form with an imperative connotation. 'Command,' (Psalms 44:5) uses tzaveh - the grammatical imperative. In our case tzav et aharon ('Command Aaron!') uses the shortened imperative. It seems that Rashi and the Sifra perceive the extra shortening of the verb as adding to the sense of urgency. Hizekuni in his commentary to this verse gives additional references to the shortened imperative. He goes further and interprets these as having a sense both of urgency and continuity.

However Be'er Rechovot (one of the two parts of Sefer Dikdukei Rashi) writes that Rashi's understanding that the command was continuous is derived from the previous verse where the word leimor is the infinitive form. He points out that this is a verb that has no specific time. For support he refers to Tosefot Yom Tov (Pesachim end of Ch. 2, s.v. lo bemashkin). He in turn refers to Rambam, (commentary on Mishna Bava Metziah 30b) where Rambam discusses the timelessness of the infinitive form, and to the Ramban (Exod. 6:10) where the Ramban argues against those who interpret leimor as meaning 'to say so,' and argues that it means 'saying' without a sense of time.

ma nishtanah halayla hazeh (Mishna Pesachim 116a) ('How did this night become different?) Being part of a Mishna it should not be surprising to find that nishtanah is Mishnaic Hebrew. In the Gemara this variety of Hebrew is called leshon chachamim ('the language of the Sages'). For the beginner who may wonder how different these varieties are, it should be made clear that speakers of Mishnaic Hebrew could understand the Hebrew of the Torah. We even find a discussion of a point of difference between the two varieties in the Gemara (Chulin 137b). R' Shabetai Sofer comments that nishtanah is a combination of the Nifal and Hitpael conjugations (Commentary on Haggada s.v. ma nishtanah). R' Shabetai Sofer described it this way because he judged all grammatical issues by criteria of the language of the Torah. Though I would not dare to disagree with R' Shabetai Sofer, who is one of the major authorities on Hebrew Grammar, I believe his point can be generalized. In Mishnaic Hebrew there is a conjugation called Nitpa'el, which is a combination of the Nifal and Hitpa'el conjugations - nishtanah is one instance of the Nitpa'el. Further examples are nistapchah (Ketubot 1:6) ('it was swept away'), nitnasah (Avot 5:3) ('he was tested'), nitmaleit (Bava Batra 5:8) ('it was filled'). R' Shabetai Sofer also points out that nishtanah is past tense. This supports the above somewhat unusual translation. It also fits in nicely with the well-documented view that originally it was the father who asked these questions of the son 'who didn't know how to ask' (Rambaam, Hil Chametz umatza, 8:2 and others, see Goldschmidt, E. D. The Passover Haggadah: its Sources and History, Bialik Institute, Jerusalem, 1960).