还 / 更【hái / gèng】

In a 比 sentence, 更 and 还 both go before the adjective and both mean "even more". 更 states the comparison; 还 adds the speaker's surprise or feeling.

还 for surprise and exaggeration

With 还, B was already high, and A going past it is unexpected. That makes 还 the word for vivid exaggeration, where the comparison isn't meant literally. 还要 strengthens it further.

更 can describe the same facts but sounds measured, the way a report would put it.

Without 比, only 更 compares

更 works on its own when the standard is clear from context. 还 on its own before an adjective means "still" or "fairly", not "more".

• × 他很高，他哥哥还高 → 他很高，他哥哥更高

With a verb of liking

更 goes before 喜欢, 想, 愿意 and similar verbs to say "prefer" or "would rather". 还喜欢 means "still like" or "also like".

In writing, 更加 often stands in for 更, mostly before a two-syllable adjective: 更加重要. 还 has no such longer form.