This word analysis has to do with “smells”, both good (fragrant), bad (stinking) and neutral.

As is often the case, the picture is fairly clear in the early Elvish languages. The root ᴱ√NṚÞṚ has to do with sweet smells, ᴱ√YOLO with foul smells and the roots ᴱ√FAWA, ᴱ√NUFU and ᴱ√NUSU are more neutral. Here are some derivatives of those roots:

ᴱ√NṚÞṚ (or ᴱ√NṚŘṚ): ᴱQ. narda- “to smell sweetly” ([nṛθa] > [narθa] > [narða] > [narda]), nardile “fragrance”, nardilea “perfumed”; G. drith “savour, smell, flavour; suggestion, inkling” ([nṛθ] > [nriθ] > [driθ]), narth “odour”, narthiel “fragrance”, narthol “fragrant, odorous” (QL/68, GL/30, 59).

ᴱ√YOLO (later ᴱ√ƷOLO): ᴱQ. yolme “stench”, yolo- “to stink”; G. gol- “to stink”, golod “stink, stench”, golog “stinking” (QL/106; GL/41) and later ᴱN. golwad- “to stink”, golw “stink, stench”, golwai “stinking” from ʒolwe (PE13/145, 162)

ᴱ√FAWA: ᴱQ. fausta- “to get smell of”, fausta “?smell”, faustima “?odorous, able to be smelled” (the last two forms unglossed in QL); G. fau- “to smell, give off a smell”, fost “odour”, fostrin “odorous”, faust “smell, odour”, faug¹ “stinking” the last having a more unpleasant connotation to due the influence of faug² “wood smoke” (QL/37, GL/34)

ᴱ√NUFU: ᴱQ. nupte “a sniff”, nuvu- “smell, sniff at”, G. nuf- “get smell of, perceive scent of, smell (tr.)”, nufri “sense of smell” (QL/68, GL/61)

ᴱ√NUSU: ᴱQ. nus “smell”, nuste “sense of smell”, G. nûn “nose (of men only)”, nus- “to take notice, perceive”, nusimos “sagacity”, nusiol “sagacious” (QL/68, GL/61)

Of these roots, there is only evidence of the last one by the 1930s. We have ᴹQ. nusta- “to smell” (PE22/103) and the root ᴹ√NUS “sense, wisdom” (EtyAC/NUS). Given the Gnomish words nusimos “sagacity” and nusiol “sagacious”, it may be that ᴹ√NUS could still be connected to “smell”. We also have ᴹ√ÑOL “smell” with derivatives like ᴹQ. olme, N. ûl “odour” as well as intensive N. ongol “stench” (Ety/ÑOL); this is perhaps a remnant of ᴱ√YOLO/ƷOLO. These two roots seems to be our best option for a neutral “smell”, though like ᴱ√FAWA it seems that ᴹ√ÑOL has some negative connotations.

For sweet smells, we have √NE “scent” with primitive ✶_nes_ “sweet smelling” as an element in Q. alanessë, S. galenas “nicotiana, pipeweed” (PE17/100). This likely reflects Tolkien’s bias as a pipe-smoker; I personally find the smell of tobacco to be unpleasant. A bit more promising is the name Nísimaldar “Fragrant Trees” (UT/167), presumably containing nísima “fragrant” which is perhaps from a verb *nis- “*to smell sweetly”. This could be from either √NIS or √NITH. The former is closer to ✶_nes_ but conflicts with √NIS “woman”. On the other hand, √NITH is closer to ᴱ√NṚÞṚ and would produce more useful Sindarin derivatives.

There is also S. lissuin “a fragrant flower” and Q. vardarianna “fragrant evergreen tree” (both from UT), but I think both of these are descriptive of the species of plants rather than etymologies of the words.

Most of the later words for foul smells are connected in some way to the name Sauron. From the Etymologies there is ᴹ√THUS with derivatives N. thost “smell”, thosta- “to stink”, thû “stench” (as in Sauron’s other name from this period: N. Thû) plus ᴹQ. saura “foul, evil-smelling, putrid” (Ety/THUS, EtyAC/THUS). The last of these reappears in the 1950s as derivatives of either √THAW or √SAW with a variety of meanings: “stinking, foul, evil” (PE17/68), “foul, vile” (PE17/183) or “cruel” (PE17/184). In one place √THAW is said to ultimately derive from √THOW “stink” (PE17/68); this derivation has the strongest connection to foul-smelling.

√THOW “stink” is one of later roots used to derive Sauron’s other name S. Thû, but it was also derived from the root √THU “puff, blow”, apparently in the sense “*foul wind” (PE17/68, 99, 124). Later, Sauron was given a new Sindarin name: S. Gorthaur “Mist of Fear” (PE17/183), from the extended root √THUS (< √THU), which was given a new gloss “evil mist, fog, darkness” (PE17/183, 187).

Tolkien seems to have ultimately abandoned the connection between stench and Sauron’s name, deciding it had more to do with cruelty instead. In the Silmarillion he was given the epithet “the Cruel” (S/32) and in a letter from the 1960s Tolkien said his name was related to a primitive adjective θaurā “detestable” (Let/380). This means that for “stench” we are left with a variety of (possibly rejected) roots of unclear meaning.

In light of the later meanings of Sauron’s name, its probably best to assume √THAW means “cruel” rather than “stink”. The root √SAW has a number of useful derivative worth preserving, but its best gloss seems to be “digusting, foul, vile” and its derivatives have more to do with “filth” and “putrescence” than stench per se. √THOW “stench” has no useful derivatives of its own and it would merge with √THAW “cruel” in almost all cases anyway. This leaves us with ᴹ√THUS from the Etymologies, if we assume that its later gloss “evil mist, fog, darkness” was a transient idea.

We are thus left with the following roots for pleasant and unpleasant smells:

*√NITH: fragrant
ᴹ√NUS: sense, wisdom; *smell
ᴹ√ÑOL: smell
ᴹ√THUS: stench

√NE “scent” might be useful, except it has no derivatives other than words for tobacco. For now I am leaving it out because there are simply too many words it might come into conflict with.