DISCLAIMER: This article is preliminary research on the part of its author (Paul Strack) and does not necessarily reflect the opinions of the owner of this site. Since the source material is complex and its interpretation can be subjective, multiple conclusions are possible.
Comparison is Sindarin is a bit tricky. We have what appears to an intensive or superlative prefix ro- (< rau-) as in rovaed, which may mean “*very shapely” or “*most shapely” (PE17/147); this suffix seems to be the equivalent of Q. ar(i)- which elsewhere was described as “virtually superlative” (PE17/56). We also have hints of a comparative prefix in words like einior “elder”. Probably the initial element of this word is related to the Quenya intensive prefix an-, with the initial ein- developing via i-affection/intrusion from an- + iaur “old”. A Sindarin intensive an- might also be seen in names like Anfauglith “Gasping Dust”, with [N.] faug “thirsty” intensified to “gasping”.
The Noldorin form of this intensive prefix was described somewhat differently in The Etymologies of the 1930s. Here the intensive prefix is given as N. a- with “dynamic lengthening” (EtyAC/N) with two examples: afarch “very dry” (< appharkha) = a- + parch and angol “stench” (< aññol) = a- + ÑOL. At this conceptual stage, the consonant a- induced a lengthened initial consonant rather than nasal mutation, but in most cases the result is the same. For example: an+p- > amph- > aff- produces the same result as app- > apph- > aff-.
Conceptual Development: Both Gnomish and Early Noldorin had well developed systems of comparative suffixes. In the Gnomish Grammar of the 1910s, the suffixes were augmentative -odron/-oglon “*more” and diminutive -inthir “*less”, with superlative variants -onta “*most” and -inci “*least” (GG/16). In the Early Noldorin Grammar the comparative/superlative suffixes were -iad/-iant, often with modified adjective stems: beleg “great” → beliad “*greater”, beliant “*greatest” (PE13/125). Neither of these systems are compatible with Sindarin.
Neo-Sindarin: Thorsten Renk proposed a system of Sindarin comparatives in his book Pedin Edhellen: a Sindarin-Course (PESC/45), and I recommend using something similar. For the intensive, he recommends prefixing an- with nasal mutation of the following consonant, basically behaving like the plural definite article in, so an + tanc “firm” = athanc “very firm, firmer”. It would assimilate to following labials as usual, so an- + beleg “strong” = ammeleg “very strong, stronger”. Before adjectives with i, Renk recommends i-affection for the prefix, so that an- + cîr “new” = echir “very new, newer”. Before words beginning with consonantal i-, he recommends intrusion producing ein- (as in einior “elder”), so an- + iaun “wide” = einion “very wide, wider”.
Renk recommends using ro- as another type of intensive, but I think it is better to treat it as a superlative, as suggested by Fiona Jallings in her book, A Fan’s Guide to Neo-Sindarin (FGNS/160); this prefix induces soft mutation. Thus rodanc “firmest”, roveleg “strongest”, rogir “newest”. I would use this superlative more or less like an ordinary adjective, as in: Galadriel i edhel rohael “Galadriel (is) the wisest (ro- + sael) elf”.
For comparing two things, Renk suggested using [N.] athan “beyond” as in Aragorn beleg athan Boromir “Aragorn (is) strong beyond Boromir (= stronger than Boromir)”. This system is analogous to Quenya comparitives which likewise use a preposition meaning “beyond” (PE17/90). I think this comparison can be intensified by using an intensive prefix, hence: Aragorn ammeleg athan Boromir = “Aragorn (is) much stronger than Boromir”.
Renk’s system is largely guesswork, but it’s probably the best we can do until we get more information.