Really
Truly
adverb
本当 is reality or truth (level 5 vocab). If you add on に, this changes the word to an adverb, and we get really or truly. 本当に!
ほんとうに
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. It shares the reading with the level 5 本当 as well, but just has a particle on the end. Really!
日本の人たちは、本当にやさしいです。
Japanese people are really kind.
このせいひん、本当に高くなっちゃったねー。
This product has gotten truly expensive.
本当にこの道であっていますか？
Is this really the way?