The prefix 'En

The prefix 'en' means 'into or in', which gives rise to several derivative words, but the curious thing is that if the prefix is attached to a word that starts with b, m and p, the consonant n in this prefix is replaced by the consonant 'm'!
Some examples are given below:

• enslave - make into slaves
• endanger - put into danger
• enable - make somebody able to do a task
• encourage - give courage to a person
• enfeeble - make a person weak
• enrol/enroll - put someone on the rolls 
• encircle - to surround
• environs- surroundings
• embrace - give a hug
• empower - give some the power 
• embitter - make someone's nature bitter
• employer - person who puts another in a job

and so on!

Knuckle Down

Believe it or not, this phrase has its origin in that childhood game of marbles

This is how you place your hand when you were about to shoot a marble at another one. The idiom means getting on to the job and doing it.
You can see the knuckles touching the ground, which is where the phrase came from. 

Peculiar Plurals

Now, in general, we know that to make the plural form of a noun we add either an 's' or an 'es' to the singular. But we have some rather special nouns that don't follow this rule:
Singular                   Plural
man                          men
child                        children
woman                    women
ox                            oxen
mouse                      mice
louse                        lice
goose                       geese
tooth                        teeth
foot                          feet
cod                           cod
trout                         trout
salmon                     salmon
sheep                       sheep
deer                         deer
oasis                        oases
radius                      radii
bacterium                bacteria
antenna                   antennae
focus                       foci
syllabus                   syllabi  or syllabuses
terminus                  termini  or terminuses
alumnus                  alumni

This is not a complete list, but it lists some of the more common plurals that are a bit strange

Participles

So, have you been properly introduced to a Participle?
It is a word that is partly a verb and partly an adjective. That's the simplest definition. 

If you want a complicated one here it is - "A participle is that form of a Verb that partakes of the nature both of a Verb and of an adjective".
There are 3 kinds of participles:-
Present Participles:-
We met a girl carrying a bunch of flowers.
Loudly banging on the door, the policeman asked to be let in.
The gazelle, realising that the cheetah was spent, trotted off to safety.
and 
Past Participles:-
Terror-stricken, he fled the cave.
Driven by ambition, he toiled day and night to achieve his aim.
We saw a convoy of military trucks drive by, laden with ammunition.

Perfect Participles:-
This represents an action completed sometime in the past. 
Having eaten, we took a short nap. 

The case of the confusing carbuncle

The word carbuncle is a really strange word! It is strange, not because it has 2 meanings , several words in English have 2 meanings- but it is strange in the complete diversity of meanings of the word!!
One meaning is - a bright red gem, in particular a garnet cut en cabochon.
The other is - a multiple boil or severe abscess on the skin - ew!

So be careful how you use the word, you might sound very wierd if the context does not make the meaning crystal clear !!!

Parenthetical Expressions

These are expressions, bounded by commas, which give clarity to a sentence. 
A example is :
" Yoghurt, on the other hand, is a fine substitute for ice-cream".

Some other common parenthetical expressions are :
after all
by the way
for example
however
in my opinion
naturally
incidentally
of course
on the contrary
on balance
to tell you the truth
indeed
in fact
honestly speaking
bluntly speaking
in short
Yes, we have all heard them and used them; this is their proper name!

'Ness'

Not talking about the LOCH NESS monster😀 

The suffix 'ness' indicates 'a state of' when it forms a word, for example:
'weightlessness' derived from the adjective 'weightless'
'happiness' derived from the adjective 'happy'
'sadness' derived from the adjective 'sad'
'firmness' derived from the adjective 'firm'
'calmness' derived from the adjective 'calm'
and so on.
The first example in this list is an example of suffixes used in tandem, the first suffix is 'less' and the second is 'ness'.
There are several words like this:
'Listlessness' derived from 'listless'
'Restlessness' derived from 'restless'
'Carelessness' derived from 'careless', etc.