The following spelling rule explains that likely is spelled likely rather than likly.

A. When the suffix ends with a consonant, keep the "e" on the main word.
B. When the suffix begins with a consonant, keep the "e" on the main word.
C. When the main word ends in "e", keep the "e" and add "ly" (I don't think this one is right myself.)
D. When the main words end in a consonant then a vowel, keep the "e" and add "ly"

When the main words ends in "e," we keep the vowel  "e" and add the suffix  "ly."

Explanation:

As per the spelling rule  

1) We just add -ly to words ending in a consonant.

For example:Slow+ly=Slowly,Week+ly=Weekly

2) We add -ly to words ending in -ful,

For Example: Ful+ly  -fully,beautiful + ly = beautifully

3) We add -ly to words ending in -l

For example: accidental + ly = accidentally ,cool + ly = coolly

4) We keep the 'e'

For Example: complete + ly = completely ,definite + ly = definitely

However, there is an exemption to this rule.

For example: true = truly

In this case, we drop  the letter "e" and add "ly."

we drop the 'e' in duly due + ly = duly

modal adverbs are used to modify specific verbs that consist of a linking verb (verb of being) and sometimes another verb. examples of modal verbs include can, could, may, might, must, shall, should, will and would.

modal auxiliary. an auxiliary verb characteristically used with other verbs to express mood, aspect, or tense. in english, the most common modal auxiliaries are can, could, may, might, must, ought, shall, should, will, and would. also called modal .

id say it means hes fearful and/or cautious

have a good day