high vs highly

High works as both an adjective and an adverb, and in its adverb use it stays physical — it tells you about position, altitude or level. Highly is an adverb only, and it has drifted almost entirely into the figurative: it means very or to a great degree.

How to use high

As an adjective: "a high ceiling", "high blood pressure", "high standards", "high prices".

As an adverb, describing where or how far up:

• "The gulls circled high above the harbour."
• "She jumped higher than anyone expected."
• "Aim high."

How to use highly

Highly intensifies adjectives, participles and a small set of verbs:

• "He's a highly experienced surgeon."
• "That outcome is highly unlikely."
• "Her work is highly regarded in the field."
• "I think very highly of her."

Notice the company it keeps: highly likely, highly skilled, highly paid, highly recommended, highly critical, highly sensitive. Nothing in that list is about altitude.

The test

Try replacing the word with very. If very fits, you want highly: "very unlikely" works, so it is "highly unlikely". Then try replacing it with far up or at a great height. If that fits, you want high: "the balloon rose far up" works, so it is "the balloon rose high".

"The plane flew highly" fails both tests once you try them — very makes no sense there, and the meaning is plainly about altitude.

The overlap: high-paid or highly paid?

A few compounds allow both forms, and the choice is idiomatic rather than logical. Highly paid is far commoner in edited writing than high-paid. Similarly, prefer highly skilled, highly qualified and highly trained.

Some fixed phrases keep high even where the sense is figurative, because they are set expressions: aim high, ride high, hold your head high, search high and low, live high on the hog. Do not correct these to highly.

Two useful contrasts to hold in mind: "prices are high" (level), "prices are highly volatile" (degree); "he set the bar high" (position), "he is highly ambitious" (degree).