Most students taking Japanese classes will learn the word 確かに（たしかに //certainly; surely） relatively early on in their studies.

It's a mega-useful word, and it shows up in conversations like this:

A:
勉強すれば合格するでしょう。
べんきょう　すれば　ごうかく　する　でしょう。
If you study, you'll pass (the test).
Literally: "studying + if do + passing + do + でしょう."
Note: If this でしょう is confusing, check out Lessons #85, 86, 87, 88, 89, & 90.

B:
確かに。
たしかに。
That's true. // You're right.
Literally: "certainly."

I probably learned that during Year #1 of my studies.

But I didn't learn the following until Year #3 or #4, sadly:

Dropping the に off of たしかに changes the meaning completely...

確かに
たしかに
surely; certainly

たしか
if I'm not mistaken; if I remember correctly

Nowadays, I use たしか much more often than I use 確かに... maybe because I'm never 100% sure about anything in life *_*.

Oh, by the way, you'll see people write 確かに with kanji or simply with hiragana as たしかに. I'm guessing that the same is true of たしか, but I've never actually seen it written--it's more of a spoken word.

Let's enter example city...

たしかに Examples

たしかにここに置いたはずなのに見当たらない！
たしかに　ここ　に　おいた　はず　なのに　みあたらない！
I'm certain that I left it right here, but I can't find it!
Literally: "certainly + here + に + placed / left + はず + なのに + not find!"
Note: はず usually gets translated to something like "should" or "must be." It's used to express an expectation that something took place or should be a certain way... and it "should" have its very own lesson. ^_^. （な）のに, in this case, means "but."

女の人の「助けて」って声がたしかに聞こえたんです。
おんなのひと　の　「たすけて」　って　こえ　が　たしかに　きこえた　ん　です。
I'm certain that I heard a woman's voice saying "Help me."
Literally: "woman + の + 'help me / save me' + って + voice + が + certainly + heard + んです."
Note: This sounds like the speaker is talking to a police officer investigating some sort of crime... or perhaps making a call to the police.

たしかに受け取りました。
たしかに　うけとりました。
I acknowledge the receipt of this item.
Literally: "certainly + received."
Note: This is basically a set phrase, and it's used in many business situations. You'll find that in formal Japanese, it's quite common to verbally acknowledge actions. For example, at many stores in Japan, if you pay for something with a 10,000 yen bill, they'll say, "I received 10,000 yen..." which is a bit like saying "out of 10,000 yen..." only they use phrases like that in tons of situations. This is one of them, and it can be used for a variety of items. If you look at these example sentences from Weblio, you'll see that Japanese people come up with all kinds of strange English phrases to match up with this Japanese. The nuance is pretty much always lost, unfortunately.

たしかに安いけど、それでも欲しくない。
たしかに　やすい　けど、　それでも　もしくない。
Yeah, it's definitely cheap, but I still don't want it.
Literally: "certainly + cheap + but + nevertheless / even so + is not wanted."

A:
このレストラン値段の割に微妙じゃない？
この　レストラン　ねだん　の　わりに　びみょう　じゃ　ない？
This restaurant doesn't quit live up to its prices, does it?
Literally: "this + restaurant + price + の + comparatively / in proportion + doubtful / questionable + is not?"
Note: 割に and 微妙 are both extremely common words, and I highly recommend memorizing both. Saying that something is 微妙 is never really a good thing. In this case, the restaurant is 微妙（びみょう / doubtful; not quite right） in comparison to （割に《わりに》） the price.

B:
たしかに。
You're right. // That's true.

たしか Examples

これはたしか渋谷で買ったと思う。
これ　は　たしか　しぶや　で　かった　と　おもう。
If I'm not mistaken, I bought this in Shibuya.
Literally: "this + は + if I'm not mistaken + Shibuya + で + bought + と + think."
Note: "This" is probably a piece of clothing or a bag. The listener probably asked where the speaker got it.

お姉ちゃん、たしか新しいパソコン欲しがってたなあ。
おねえちゃん、　たしか　あたらしい　パソコン　ほしがってた　なあ。
If I remember correctly, she [my sister] was saying that she wanted a new computer.
Literally: "older sister + if I'm not mistaken + new + PC + was acting like she wanted + なあ."
Note: The speaker is probably talking to other family members or friends as they discuss what present they should get the speaker's older sister for her birthday or something. Also for another lesson is this use of the suffix ～がる, which means "acting like." It is often attached to 欲しい, "wanting." So use 欲しい（ほしい） when talking about yourself and 欲しがる（ほしがる） when talking about other people.

あの二人、たしか去年結婚したはずだよ。
あの　ふたり、　たしか　きょねん　けっこん　した　はず　だ　よ。
If I'm not mistaken, those two got married last year.
Literally: "those + two people, + if I'm not mistaken + last year + marriage / wedding + did + はず + is + よ."
Note: This would also work fine without はず (so, removing はずだ), and the meaning wouldn't change all that much. But with はず, the speaker sounds just slightly less certain of the information. As in, they have not gotten confirmation as to whether these two got married or not... but presumably they did. Nuances like this are tricky, so I wouldn't worry about them too much.

A:
飲み会何時からだっけ？
のみかい　なんじ　から　だっけ？
What time does the (drinking) party start, again?
Literally: "drinking party + what time + from + is + っけ？"
Note: Adding っけ？ is a lot like saying "again?" at the end of the sentence. The speaker is indicating that this information has slipped their mind.

B:
たしか6時から。
たしか　ろくじ　から。
If I remember correctly, it's at six.
Literally: "If I'm not mistaken + six o'clock + from."