words

What are the only four commonly used words in the English language ending in 'dous'?

(There is a 5th not-so-common word, and a 6th very uncommon one, which also has one of each vowel in the correct order)

Tremendous, horrendous, stupendous and hazardous.

The not-so-common word is jeopardous.

The uncommon word with all the vowels in the correct order is Annelidous (pertaining to annelid, which is a segmented worm).

What is the only word in the English language that ends in 'mt'?

Dreamt.
What are the longest one-syllable words in the English language?

Longest single-syllable words questions often appear in quizzes and trivia pages. The only single-syllable, ten-letter word we know is scraunched

which is the past tense of the verb to scraunch, meaning to make a crunching noise, for example when walking on gravel.

There are several single-syllable words with nine-letters, strangely most beginning with the letter'S':

screeched, strengths, straights, scratched stretched, scrounged, scrunched, and the less commonly-known: scraughed, scraunched, scrinched, scritched, scrooched, sprainged, spreathed, throughed and thrutched.

Words that sound exactly the same and have opposite meanings. (They are spelt differently but phonetically - they sound - the same):

Here are several examples:

Raise (build up) and Raze (destroy to the ground).
Wave (dismiss) and waive (allow).
Sent and Scent (as in 'dispatch to' and 'receive from' - purfume)

Pair and Pare (to increase by doubling-up, and to reduce by cutting or trimming away ).

Words which without changing the spelling or sound have two opposite meanings -

There are a surprising number examples of words spelled the same which have two opposite meanings.

These words are called autoantonyms, or contranyms or antagonyms.

Here are some very common autoantonym (or contranym or antagonym) words:

Outstanding (satisfactory - standard exceeded) and Outstanding (unsatisfactory - standard not met).
Oversight (check, monitor) and Oversight (neglect, forget)
Weather (endure - stand test of time or resist storm or pressure) and Weather (erode - wear down or denude).

Clip (join two or more things together as with a paper-clip) and Clip (divide something into two or more pieces, as in clip an article from the paper or clip someone's hair).
Dust (remove a layer of powdery substance) and Dust (apply a layer of powdery substance, as in dusting crops or dusting for finger-prints).
Trim (add to or embellish, as in trim the Christmas tree) and Trim (cut away something, as in trim someone's hear or a hedge).
Cleave (split apart or break) and Cleave (stick or adhere).

Ravish (to violently abuse) and Ravish (to delight)
Fast (quick) and Fast (stuck tight)
Sanction (a permission) and Sanction (a preventative penalty)
Sanguine (cheerful) and Sanguine (bloodthirsty)
Bolt (secure in place) and Bolt (run away).
Garnish (add to - embellish or decorate) Garnish (remove from - as in legally serving notice to seize money or assets).

Bound (fixed) and Bound (moving, as in travelling).
Left (gone) and Left (remaining).
Mad (angry about) and Mad (attracted to).
Livid (angry) and Livid (pallid - lacking colour and spirit).

Wind-up (start something, like a clock or an argument) and Wind-up (finish something, like proceedings or a talk).
Blow up (inflate - create - a balloon) and Blow up (destroy with explosives)