Tafsir of Ya seen 36:4

(Upon a straight path) is a second predicate for "indeed," and al-Zajjaj chose this, saying: "It is the best [option] in Arabic." Alternatively, it is a state (hal) of the pronoun of the Prophet (peace and blessings be upon him) latent in the prepositional phrase, or it is a state of the noun of "indeed" based on the opinion of those who permit the state to be derived from the subject (mubtada'). It is also permissible for it to be related to "the messengers," and the intended meaning is not the state [of them at that moment] or the future; rather, [it means] "from among those who were sent upon a straight path." It may also be a state of the referent of the relative pronoun that is latent in the active participle, or a state of the negated [or qualified] "messengers."

Al-Zamakhshari mentioned none of these aspects except for it being a predicate and a linkage (silah) for "the messengers." Regardless, the intended meaning of "the straight path" is that which encompasses true beliefs and laws. The purpose of the information is not to inform [the listener] by distinguishing those who were sent upon a straight path from others who are not of that quality—so that it might be said this was already known, as everyone knows that messengers can only be upon a straight path—but rather the purpose is to announce that he is described as such, and that what this person who is described as such has brought is for the glorification of their status. Thus, they pursued this path in a manner of brevity.

Furthermore, the indefinite form in "a path" is for glorification; it indicates that he was sent from among the straight paths upon a path whose description cannot be fathomed, and this is something that was not known before. It is not an objection that the straight path is but one—do you not see the saying of the Exalted: "Follow it, and do not follow [other] ways"—because for every prophet with a law, there is a method that is straight. When considering the return to the Sender (Exalted is His Majesty), they are all unified, but when considering the specificity of the messenger and the laws, they differ; thus, it is valid to say he was sent from among the straight paths, and so on. Furthermore, it is a supposition, and the supposition is the glorification of this path—that there is no path more upright than it, whether in reality or by supposition, and there is no regard for whether there is another one there or not. This is close to the stylistic construction "Someone like you does not do such-and-such," so understand this and do not be heedless.