(ほんとう)

ほんとうに
adverb
really, truly
1. really, truly
Used to emphasize that something is genuine or to express sincere feelings.
本当(ほんとう)ありがとう
Thank you so much.
This movie was really interesting.
He is really good at Japanese — the first time I heard him speak, I thought he was Japanese.
2. indeed, certainly
Used to confirm or agree with something strongly.
本当(ほんとう)そう(おも)
I really do think so.
今日(きょう)(あつ)。」「本当(ほんとう)に。」
"It's hot today, isn't it?" "It sure is."
That was really tough, wasn't it. Good work.

本当(ほんとう)に is the adverbial form of 本当(ほんとう) (truth/reality), used to emphasize sincerity or degree.

CASUAL FORMS:

COMMON USES:

SIMILAR ADVERBS:

NOTE:
本当(ほんとう)に emphasizes both degree (really good) and genuineness (truly/sincerely), while とても only emphasizes degree.