Tafsir of Al-Hijr 15:7

The word "lawmā" (لوما), like "lawlā" (لولا), is used in one of two senses: the abstention of a thing due to the presence of another (implication), or incitement (tahdīd). When the latter is intended, it is followed only by a verb, whether explicit or implied. When the former is intended, it is followed only by an explicit noun, or one that is implied according to the Basran school. An example of this is the verse of Ibn Muqbil:

"Were it not for life, and were it not for religion, I would have reproached you for some of what is in you, as you have reproached my defects."

Some have held that the mim in "lawmā" is a substitute for the lam in "lawlā." Examples of such substitution include istawlā and istawmā, or khālaltuhu and khālamtuhu (meaning my friend, khalī and khalmī). Al-Zamakhshari mentioned that "law" is compounded with "lā" and "mā" for two meanings, whereas "halla" is compounded only with "lā" for the purpose of incitement. Abu Hayyan, however, favored the view that both are simple (non-compounded) forms and that the mim is not a substitute for the lam. Al-Maliqi stated that "lawmā" is used only for incitement, though he is refuted by the aforementioned verse.

Regardless, the intended meaning here is incitement, i.e., "Why do you not bring us the angels," so that they may bear witness for you and support you in your warning, as in the Almighty’s saying, recounting their words: "Why is not an angel sent down to him, so that he may be with him as a warner?" Or, they might punish you for your rejection, just as [divine punishment] used to come to the nations that denied their messengers.

In your claim, for there is no doubt regarding the Almighty’s power to do so, nor regarding your need for it to facilitate your mission, as we will not believe you in this grave matter without it. Alternatively, it may mean: if you are indeed among those truthful messengers whose nations that rejected them were punished.