How to use the words ending with "-ly"?

Adverbs in English are interesting from the perspective of a Grammaticality Judgement Test — if you ask 100 people in the street to say whether or not the following sentences are correct, you can pretty much reach a consensus:

• Quickly, I ate bread.
• I quickly ate bread.
• *I ate quickly bread.
• I ate bread quickly.

So it appears that in English, some adverbs can appear before the main clause, after the main clause, and between the subject and the verb — but not between the verb and the object.

This is an easy example, but you can do similar tests with more complex forms (like playing with adverbs in the present perfect, for instance) to determine grammaticality, and determine overlap, and hopefully figure out where a "safe" place for adverbs is.

• Quickly, I have been eating bread.
• I quickly have been eating bread.
• I have quickly been eating bread.
• I have been quickly eating bread.
• *I have been eating quickly bread.
• I have been eating bread quickly.

Again, we see that English accepts adverbs in pretty much any position except for between the main verb and the object. Other constructions, such as SUBJECT-AUXVERB-AUXVERB-ADVERB-MAINVERB-OBJECT might be unusual, but most native speakers would not say that "I have been quickly eating bread" is incorrect, so much as odd... though acceptable. "I have been eating quickly bread," however, would be almost universally seen as being grammatically incorrect.

My advice: avoid sticking an adverb between the main verb and the object, and the odds are good that you'll be okay.

N.B.: Compared to some other languages, English allows for a lot of different adverb placements. German, for instance, is much more inflexible regarding adverb placement.

Almost all words ending in -ly are adverbs, because adding an -ly to most adjectives (e.g. quick) transforms them into adverbs (e.g. quickly). You can use them as you would any adverb; they are absolutely valid words.

I went quickly down the street in search of my dog.

However, there is a small group of adjectives which end in -ly, such as gentlemanly. Most of these are nouns (e.g. gentleman) with an -ly ending. In these cases, instead of turning them into adverbs, the -ly ending has the meaning of like (i.e. gentlemanly means like a gentleman).

That was a gentlemanly action.