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^{This is an entry in the Fortnightly Topic Challenge #41: Short and Sweet.}

English spelling is very strange. Sometimes you encounter words with letter combinations that would seem entirely unpronounceable on their own.

I have listed a few such letter combinations below. Try to determine what word each combination is from.

• The letters may appear at the beginning, middle or end of the word, but they are all consecutive (no other letters appear in between)


• All words will be found in any reasonable dictionary. None are particularly obscure.


• In cases where there may be 2 or 3 valid answers, an additional letter has been supplied to disambiguate. The additional letter must be found somewhere in the answer word.


• Since using computers makes this challenge trivial, please try to work this one out using only your own brain (or those of your friends).

GNAG
PSTR (U)
KWH
NDTHS
GHTH (U)
CST
WKW
MBSTR (C)
CKKN (F)
RWAT (U)
NDTHR
LFTH
RTGA
XACE
APK
ESHOL

When you have all the words, their initials will spell something that English in general, or these words particularly, could use.

The final answer is

SUBTLE ADJUSTMENT


(note that subtle contains the rather rare letter combination BTL. Not sure if that is on purpose ...)

formed by the words

- Signage (Parseltongue)

- Upstream (Parseltongue)

- Buckwheat (Parseltongue)

- Thousandths (Parseltongue)

- Lighthouse (JonMark Perry)

- Ecstatic (Parseltongue)


- Awkward (Dorrulf)

- Dumbstruck (me, kazi0)

- Jackknife (me)

- Underwater (Dorrulf)

- Spendthrift (kazi0)

- Twelfth (me, kazi0)

- Mortgage (Dorrulf)

- Exacerbate (kazi0)

- Napkin (kazi0)

- Threshold (Dorrulf)

_{I tried to reconstruct who came up with individual words first; which is kind of hard given the timelines and grace period edits. Apologies for any mistakes.}

But if you ask me (non-native speaker here):

English spelling in general can use a not-so subtle adjustment.

I found some remaining ones:

MBSTR:

duMBSTRuCk

CKKN:

jaCKKNiFe

LFTH:

tweLFTH

Partial answers so far:

1. Signage
2. Upstream
3. Buckwheat
4. Thousandths
5. Eighth (not quite correct, due to disambiguation)
6. Ecstatic
7.
8.
9.
10. Overwatch (not quite correct, due to disambiguation).
11.
12.
13.
14.
15.
16.
17.
18.
19.
20

Partial:

1.

2. UPSTREAM

3.

4. THOUSANDTHS

5.

6.

7. AWKWARD

8.

9. ___CKKNIFE

10. UNDERWATER

11.

12.

13. MORTGAGE

14.

15.

16. THRESHOLD

Some of the words that haven't been found yet

MBSTR (C):

dumbstruck

NDTHR:

spendthrift

LFTH:

twelfth

XACE:

exacerbate

APK:

napkin

5:

LIGHTHOUSE (thanks to the U)