Infinitive is the nonfinite form of verb, which means that it doesn't require subject or object (but may have).

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 it resembles Latin's Gerundive.

In Nûrlâm there is only one form of infinitive, which is formed by adding suffix -ut to the verb's stem. Some modal verbs, such as “maug” (must) or “gâkh” (may), do not have infinitive form. Additional forms of English Infinitive (passive, perfect, progressive) are technically a clauses with auxiliary verbs and infinitives, and being translated as such. Moreover such complex forms of infinitive usually express some irrealis (not indicative) modalities or used together with impersonal parenthesis which may be transformed into more simple form with relative clauses. However suffixes of aspect may be added in Nûrlâm (for example -ûk, thus resembling active perfect infinitive), and simple passive infinitive may be expressed with gerundive. Examples:

• “He seems to have done his job” = “It seems, he have done his job” ⇒ “Bozdâ takrampuz bul takob” (infinitive is not used at all in Nûrlâm)
• “He appears to have reported already” = “It turned out that he have reported already” ⇒ “Thakuzâtûk zamash tagauthuz dok” (again, no infinitive);
• “Orc was glad to have killed elves before” ⇒ “Ash uruk kuz glaz dogutûk golug dok”; but for better clarity it's better render the phrase as “Orc was glad that he killed elves before” ⇒ “Ash uruk kuz glaz zamash tadoguz golug dok”;
• “I expect this job to be done tomorrow” ⇒ “Dadarb za bul krampat ârshab” (Nûrlâm uses gerundive instead of infinitive to show passive meaning)

Infinitive may take almost any role in the sentence:

• Part of complex predicate (predicative): “I like to kill elves” = “Dabrogb dogut golughai”;
• Subject (counted as 3rd person singular): “To live is to die” = “Kîbut kulâ matut”, “To spare an enemy is not proper orcish behavior” = “Kaskut ash goth kulâ narthan okturm urukhaib”;
• Object: “To live is to die” = “Kîbut kulâ matut”;

Infinitive is rarely used in Nûrlâm as modifier, adverbial and after verb “to be”, as the Gerundive is often used instead (as in Ring Inscription). Infinitive in Nûrlâm may replace English gerund (despite the term “gerundive” seeming similar). Infinitive can take other words to form a non-finite clause. Pronominal objective suffixes can be attached to infinitives, the same way as to gerundives in Ring Inscription (e.g. krimputul = to bind them).

Unlike many languages, the infinitive is not a dictionary form of verbs in Nûrlâm, with Imperative/Jussive being such form. However jussive form (without suffix -ut) is sometimes called “bare infinitive”. But this term should not be confused with English bare infinitive. Modal verbs in Nûrlâm always require full infinitive (with suffix -ut) as their predicative, while bare infinitive of Nûrlâm is restricted to few modalities.

Nûrlâm's author suggests the reason of adopting gerundive suffix -at for infinitive in late orcish dialects (as Svartiska or Shadowlandian). Because of similarity gerundives and infinitives were often interchangeable in colloquial speech. As gerundives were used more frequently, in Modern Nûrlâm infinitives took gerundive's ending -at and its functions, but combined grammatical form is called infinitive.

Here is comparison chart summarizing distinctions between Infinitive and Gerundive: include:Infinitive_vs_Gerundive

1. Black Speech at French Wikipedia