"She works hard every day."
I agree. They did state in the tips and notes at one of the first few units that they'd be requiring it as a general rule, and I understand their desire for uniformity (here), but I think this sometimes does more harm than good, especially where the sentence flows naturally without it.
很 heightens the extreme of the adjective
If 努力 is used before a verb like 工作 or 学习, is it an adverb or an adjective? In English, adjectives modify nouns and adverbs modify verbs.
Being that 努力 has 2 syllables, it would also need to use 的 after it to modify 工作 as a noun. Since there is no 的 in this phrase, 工作 is likely a verb here.
努力的工作 ( adjective + 的 + noun ) = hard work (thing)
努力工作 ( adverb + verb / verb + verb) = working hard (action)
In general, you need to put 的 after a two-syllable adjective, and should not put it after a single-syllable adjective.
If 工作 is a verb, then 努力 would have to be an adverb or part of a compound verb.
I've seen a Duo exercise that used 很 + verb (e.g. 很喜欢 or "really like").
Subj. + 很 + [Certain Verbs]
Certain "psychological verbs" related to feelings or emotional sates can be modified with 很 (hěn) to increase their intensity.
Since 努力 doesn't seem to be a verb related to emotions or feelings, 很 and 努力 would both have to be adverbs. While I just learned something new about stringing 2 adverbs together, I still don't know why the 地 is missing from this phrase.
In Chinese, we place the structural particle 地 (de) after the adjective to make it function as an adverb.
你 要 努力 地 学习
Nǐ yào nǔlì de xuéxí.
You must study hard.
Anyone know why 地 wasn't used ?
Although Yellowbridge dictionary says 努力 is an adjective under "parts of speech", it also shows that it can be translated as a verb that means "to strive" or "to try hard."
As a verb, there would be no need to use a 地 between 努力 and 工作. Like compound nouns, multiple verbs can be used together without any special characters.
If “to strive" or "to try hard" is considered a psychological feeling or emotional state, then the 很 is being used to intensify the verb 努力.
Literal: She (她) every single day (每天都) really strives (很努力) to work (工作).
Natural: She works hard every day.