Learn English – Position of the adverb “of course”

When it comes to expressing consent and assurance, I'd say the difference certainly isn't very big. :-) When faced with a question like

Can I have some more coffee?

..."Of course" would probably be a more typical reply. But there's nothing wrong with "Certainly" either (although it might sound a little more formal).

Oh, as you mention the context is everyday conversation, you should also consider something like:

Yeah, sure!

:-)

Hardly belongs to that category of words in English whose meanings don't match or coincide with the first or most logical ones that pop into your head.

When hard is used as an adjective in the following: "He is a hard worker," we understand it to mean that the subject is someone who works long hours with great zeal and diligence. But English, if it needs reminding, isn't always logical, and the real meaning of hardly is surprisingly quite the opposite.

not at all; scarcely (used to qualify a statement by saying that it is true to an insignificant degree)

And what's more, hardly usually precedes the verb it modifies; therefore, the phrase "He hardly works" means someone who works very little or so rarely that it's almost not worth the effort of mentioning.

The adverbial form of the adjective hard is instead...hard.

• He is a hard worker (adj)
• He works hard (adv)
• He works really hard (adv)

Fast is another example of a word that is used as both an adjective and an adverb.

• He is a fast worker (adj)
• He works fast (adv)
• He works really fast (adv)

Typical adjectives which do not add ly to form adverbs are: early, late, hard, fast, long, high, low, deep, near.
Nowadays grammarians call them flat adverbs but in the past they were better-known as plain adverbs. Why do they exist? Surely they ought to have some visual clue as to their grammatical function? I found this brief explanation which might help (emphasis mine)

"In EMnE [Early Modern English], plain adverbs (those identical in form to adjectives) were widely used, even by careful writers. The list of acceptable plain adverbs today has shrunk to a few frequent ones, which often seem to have survived only because the corresponding form in -ly has a different meaning. We say 'I worked hard until very late' because hardly and lately do not mean the same thing as hard and late. Except for a handful of common time words like early, daily, weekly, and hourly, even adjectives that already end in -ly are at best uncomfortable when used adverbially. For example, though contemporary dictionaries still list friendly as an adverb, most of us would hesitate to write it as such, preferring a paraphrase like in a friendly manner or even the phonological monstrosity friendlily (also recognized by some dictionaries).

"Some of the common closed-list adverbs (those not derived from adjectives) of EMnE have since become obsolete or at least archaic. Examples include afore 'before,' ere long, without 'outside, out of doors.' hither, and thither." (C.M. Millward, A Biography of the English Language, 2nd ed. Harcourt Brace, 1996)

Now onto the question of where to position an adverb of manner in a sentence. When an adverb modifies a verb, you can usually put it either in front, in the middle, or at the end of a phrase. Middle can be either after the first auxiliary verb, after be as a finite verb, or before any other finite verb if there is no auxiliary verb.

• Quickly let's get back to work!
• Let's quickly get back to work.
• Let's get back to work quickly.

But avoid placing an adverb between the main verb and the object

• Quickly he reads a book (yes)
• He quickly reads a book (yes)
• He reads quickly a book (NO)
• He reads a book quickly (yes)

Really

Really is an adverb of degree, it's often used to emphasize an adjective or adverb but it can also modify a verb. Unfortunately, regarding positioning, there are no hard or fast rules.

1. He's a really hard worker (modifies the adjective, hard)
2. He really works hard (modifies the verb, work)
3. He works really hard (modifies the adverb, hard)