فَأَنذَرْتُكُمْ نَارًا تَلَظَّىٰ
The root ن-ذ-ر was taken up in the discussion of al-Baqara 2/6. In brief: indhār is the giving of advance notice of a danger to come. Two conditions lie inside the word — the danger will be real, and the notice will be given before the danger arrives.
The tense of the verb is striking. Andhartukum — perfect, past tense: "I have warned you". Something that is to happen in the future is spoken of, but the verb is past.
This is a technique of emphasis in Arabic: a matter whose occurrence is certain is told with the perfect, as though it had already happened. Here it has two layers: both that the warning has actually been given (this sūra is the warning itself), and the certainty of the thing warned of.
And the shift in the subject. In the thirteenth verse the subject was plural (ʿalaynā, lanā — We). In the fourteenth it turns to the singular: andhartu — I have warned. Then in the fifteenth it will pass to the third person. In the Qurʾān the shifting of the direction of address is called iltifāt, and it is used to draw attention.
Who is speaking in this shift has been debated: is the agent of the verb God or the Prophet? The majority say God; the context (the "We" of the two preceding verses) supports this. Some exegetes read it as the Prophet's word.
What was said about khusrin in Sūrat al-ʿAṣr holds here too: indefiniteness can convey indescribability and dread. "Such a fire as that."
لَظَىٰ occurs in the Qurʾān as one of the names of hell: "No indeed! It is Laẓā, tearing away the limbs" (al-Maʿārij 70/15-16).
The original form of the verb is تَتَلَظَّىٰ (tatalaẓẓā); one of the two tāʾs has been dropped to avoid the heaviness of their standing side by side. This lightening is often met with in the Qurʾān.
Form V (talaẓẓā) conveys reflexivity and continuance: to blaze of itself, to be all flame. The fire is not kindled from outside; it produces its own flame.
This agrees with the general logic of the sūra. In the seventh and tenth verses the outcomes arose out of the person's own act, by being made easy. Here too the fire is described as something that burns of itself.