about "smoothes"?

Question

I'm learning English.

"She smoothes her blonde hair."

Why is -es attached to the verb smooth not the suffix -s?
Why the plural form of "mouth" is "mouths" not "mouthes"?

Answer 1

Not absolutely certain, but my guess is that it stems from a variation in the pronunciation of the word in different dialects of English, each of which in their own way was captured at the time English spelling was becoming established.

First, about "mouths" -- NO noun form ending in -th adds "es" to form the plural. That's not even what's going on with "smoothes", since this is a VERB form.

Second, the post that says it is spelled "smooths" is HALF-correct. The word may be spelled with or without the e.

Now note -- "smooth" is itself a bit unusual as a word ending with a VOICED "th". In most cases, if the final th is to be voiced it is followed by an e, and so forms based on them simply add a final -s (or -d for past tense verbs). In fact, the ONLY other word I know with a final voice "th" --no e-- is "with", which some dialects pronounce voiced, some as unvoiced -- which may provide a key for this case too.

Take a quick look at other words with final -the:

* bathe-s
* breathe-s
* clothe-s
* lathe-s
* loathe-s
* scythe-s
* seethe-s
* sheathe-s
* soothe-s
* swathe-s
* teethe-s
* tithe-s
* wreathe-s
* writhe-s

Note especially the following parallel groups -
(a) noun form (unvoiced th) - bath, breath, cloth, loath, sheath, swath, teeth, wreath
(b) similar verb forms (voiced th) - bathe, breathe, clothe, loathe, sheathe, swathe, teethe, wreathe

the nouns in (a) add an -s (to end -ths) when forming the plural; in most cases the 'th' is still pronounced without the voice
the verbs in (b) add an -s (yielding -thes) for the singular endings ('he breathes' , etc)

Compare also how "soothe" endings with a voiced th, but the word "forsooth", originally from the same root, has an UN-voiced th.

Thus the addition of 'e' after a final th has become, generally, a marker for a voiced, th, and at the same time a means of distinguishing it from related or similar-looking forms with unvoiced th.

Start checking out root words that end simply end with 'th' you will see that they almost always have an UN-voiced th, and add simply an '-s' to form plurals (if they are nouns) or the 'third person singular' verb form.

Perhaps the following would explain it -- I don't have evidence of this at the moment, but I wonder whether 'smooth' was at one point, at least in some dialects, pronounced with an UN-voiced th. And it was THIS pronunciation that established the spelling. But if in the pronunciation changed -- or the Modern English pronunciation was derived from a dialect that 'voiced' it. . . and from which we also get the variant spelling of "smoothes" -- we could (did) end up with the spelling "smooth", where we should expect "smoothe".

And since, in this case (unlike those listed above) there is no related form that needs to be distinguished ("breathe" vs. "breath", etc), there was no 'pressure' to change the spelling.

Answer 2

You have spelled this verb wrong - it is "smooths."
English is irregular. You could go crazy trying to explain it. For example - why is the plural of cow "kine?"