Most adverbs are made from adjectives by adding -ly:

slow – slowly

easy – easily

However, there are many of those that have the same forms as the adjectives and take no -ly:

close, deep, fast, hard, high, late, near, still and more.

More adverbs without -ly: often, always, soon.

Some of them have two forms: close-closely, deep-deeply, hard-hardly, high-highly, late – lately, near – nearly.

Note: The meanings can be very different!

More pairs: direct (The plane goes direct to Paris.) – directly (to speak directly to a manager) , flat (to lie flatly) – flatly (to refuse flatly – ‘in a definite way’), pretty (pretty good/warm) – prettily (to dress prettily), rough (to sleep rough)– roughly (= ‘approximately’), wide (The door was wide open.) – widely (to travel widely).

Watch out! Some words ending in -ly are adjectives:
cowardly, friendly, lovely, lively, motherly.

They do not have an adverbial form and are used in adverbial phrases, e.g. in a friendly way.

Words like daily, weekly, monthly, yearly can be both adjectives and adverbs.

More -ly adjectives and adverbs with the same meaning: early, leisurely (‘without hurrying’).

Comparatives

more + -ly: more slowly, more easily

! But: earlier, louder.

Also:

hard – harder

high – higher

soon – sooner

fast – faster

well – better

bad – worse