For many years in various Neo Black Speech dialects suffix -at was used for infinitives, taken from literal translation of Ring Inscription. But on year 2007 in the issue 17 of journal “Parma Eldalamberon” it's analysis by J.R.R. Tolkien himself was published. And there was stated that -at was suffix of some sort of participle, but with another translation into gerund. So by combination of appliances as participle, gerund and infinitive it resembles Latin's Gerundive.

In Latin Gerundive is the form of verb that functions as verbal adjective. There is no full equivalent of gerundive in English. The closest translation is a clause with infinitive such as “books to be read”, combining a transitive verbs with it's object with a sense of obligation. Another translation is verb with prefix “must-” as in “must-read book”.

Nûrlâm's gerundive is neither a full equivalent of Latin's (or closer to late Latin where it was mixed with participles). The term was adopted as the closest one to Tolkien's definition of suffix -at found in Ring Inscription. Latin gerundives were used more frequently with passive meaning and as predicative. In Ring Inscription it's used as active adjectival phrase (“ring to rule them all”). Gerundive in Nûrlâm always have a sense of obligation, purpose or future action, thus changing modality of expression into some irrealis mood. Passive function of gerundive must be clarified with verb “to be” (= “kul”) and object in instrumental case (if present). Compare “durbatulûk” (to rule them all) with “kul durbat takirzi” (is to be ruled by them).

So functions of gerundive in Black Speech and it's translations include:

1. expressing intention or purpose:
   1. “for” + gerund (“ash nazg durbatulûk” ⇒ “ring for ruling them all”), in Latin gerund do not take an object, while in English it can (as well as gerundive in Nûrlâm). Please note that other uses of English gerund are not translated into gerundive of Nûrlâm, despite similar names;
   2. infinitive of purpose or Latin's supin (also expresses purpose), including expressions like “in order to”: “Gimaug khîgut skâtat Ashûk Urun ârshirzi Durinob” ⇒ “You must hurry to reach the Lonely Mountain by Durin's day” = “In order to reach the Lonely Mountain by Durin's day, you must hurry”;
   3. passive infinitive (“golug (kulâ) dogat” = “elf (is) to be killed”); should be clarified with verb “kul” (to be) or agent in instrumental case;
   4. expressions like “going to”, “about to” + infinitive: “Da(kul) dogatûk ash golug” = “I'm going to kill an elf” = “I am about to kill an elf”. May be replaced with Prospective aspect or in analytical language with verb “pant-” (to intend) + main verb in infinitive form;
2. necessity:
   1. verb “must” with passive verb/participle: “Gunduzg (kulâ) shadat ” ⇒ “Gondor must be destroyed”;
   2. necessitative modality (similar to subjunctive mood) after impersonal constructions (like “it's necessary that”, “it's important that”): “It's necessary that you be at the gates of Gondor tomorrow at dusk” ⇒ “Bolkarz fizûr kulat hûmob ik Gunduzgob shi muth ârshab”, “It's impossible that he have flown away” = “It's impossible for him to fly away” ⇒ “Narfalgarz tazûr skoiratûk”;
3. impersonal sentences (like “It's time to go” ⇒ “Il ukhat”);
4. verbal adjective (like “edible”, “adorable”, etc.), may be replaced with one of examples from point 1 (e.g. “edible fruit” = “fruit for eating” = “fruit to eat” = “gau thokhat”);
5. jussive modality (similar to imperative mood) in dependent clause after 3rd person orders using such verbs as “command”, “order”, “demand”, “insist”, “request”, “recommend”, “propose”, “suggest”: (“Morgoth demands that you bring him hobbits alive” = “Morgoth thukhûlâtaf thrakatan piraga kîbûrz”);
6. in questions after “Why” and answers to them: “Why did you kill a dragon? – To earn the reward” ⇒ “Mûr fidoguz ash lûg? – Gambat raunum”. This application is related to points 1 and 2;
7. future passive or future participle in languages that have them: “The elf will be executed” ⇒ “Golugum kubâ dûmbat”;
8. English future (perfect) continuous tense: “Dakub gorgat ash golug” ⇒ “I will have been butchering an elf”.

As it is seen in Ring Inscription, gerundives can take pronominal objective suffix (e.g. gimbatul). Gerundives do not take case endings. Gerundives rarely used as predicative in Nûrlâm except verb “kul” (to be) + gerundive, however verb “kul” is usually omitted in present tense (= “matters of taste should not be debated”, lit. “tastes are not for debating” ⇒ “shaub (kulû) nartishugbat”). Please note, as gerundive already marks the future tense, it is rarely used together with future form “kub” (= “will”) of the verb “to be”.

Because of similarity gerundives and infinitives are often interchangeable in colloquial speech. As gerundives are used more frequently, in Modern Nûrlâm infinitives take gerundive's ending -at, but combined grammatical form is called infinitive.

Here is the comparison chart summarizing distinctions between Gerundives and Infinitives:

1. Gerundive at Wikipedia
2. Black Speech at French Wikipedia