Kore-wa kitto yoku ureru-yo. (I bet this product will sell really well.)

Situation 1: Mr. Mita and his colleague Mr. Sere are talking about their firm's new selection of healthier chocolates.

三田： このチョコ、いいね。いろいろな味が楽しめるし、体にもいいし。

セレ： うん。これはきっとよく売れるよ。

Mita: Kono choko, ii-ne. Iroirona aji-ga tanoshimeru-shi, karada-ni-mo ii-shi.

Sere: Un. Kore-wa kitto yoku ureru-yo.

Mita: This chocolate's good. There's a range of great tastes to enjoy, and it's healthy too.

Sere: Yeah. I bet it'll sell really well.

Today we will introduce the proper use of the adverb きっと. きっと means "surely," and it's used to show that the speaker is confident about something. Examples: きっとうまくいくよ (I'm sure things will go well); あしたはきっといい天気(てんき)になるよ (The weather will probably be good tomorrow); ずいぶんがんばっているんだから、きっと合格(ごうかく)するよ (Because you're working so hard, I'm sure you'll pass). きっと is often used with expressions of conjecture such as だろう (will), 思(おも)う (think) or 違(ちが)いない (must), as in: 今(いま)ごろきっと二人(ふたり)はあのレストランで食事(しょくじ)をしているに違(ちが)いない (They must be having dinner at that restaurant around now). かならず is similar in meaning to きっと but is used when there are objective grounds for a statement, while きっと is used when the statement is more subjective and less certain. Moreover, かならず can't be used in negative sentences.

Situation 2: At home, Mr. Okubo tells his wife that he forgot to mail a postcard.

夫： ごめん、はがきを出し忘れちゃった。あした、 出すよ。

妻： きっとよ。忘れないでね。

Otto: Gomen, hagaki-o dashi-wasurechatta. Ashita, dasu-yo.

Tsuma: Kitto-yo. Wasurenai-de-ne.

Husband: Sorry, I forgot to mail that postcard. I'll do it tomorrow.

Tsuma: You'd better. Don't forget.

きっと is also used when the speaker wants to express a strongly felt wish or invitation to the listener, as in ニューヨークに着(つ)いたらきっとメールしてね (Make sure you mail me when you arrive in New York). きっと can also be used by itself, as in the wife's remark. When きっと refers to the speaker's action, it emphasizes their strong will to do something, as in: あしたは 区役所(くやくしょ)へ行(い)けないけど、あさってはきっと行くよ (I can't make it to the ward office tomorrow, but I'll go the day after tomorrow for sure). In general, かならず can be used in place of きっと, but かならず sounds stronger.

Bonus Dialogue: Mr. Mita has just had his heart broken and looks suitably depressed. His colleague Mr. Sere talks to him.

セレ： だいじょうぶ。そのうちきっといい人(ひと)が見(み)つかるよ。

三田： いいよ。そんな気休(きやす)めを言(い)ってくれ なくても。

セレ： 今度(こんど)、合(ごう)コンのパーティーがあるらしいから、行(い)ってみたら？ きっと楽(たの)しいよ。

三田： いいよ。きっとまたうまくいかないよ。

セレ： あれっ。今(いま)までになく悲観的(ひかんてき)だね。

三田： この前(まえ)ネットで読(よ)んだんだけど、日本 (にほんじん)の男性(だんせい)の４人(よにん)に１人(ひとり)は、そのうち一生(いっしょう)独身(どくしん)になるんだって。たぶん、ぼくもその１人になるじゃないかと思(おも)う。だから、独身でも楽しく 暮(く)らせるように、自分(じぶん)の考え方(かんがえかた)を変(か)えようと思っているんだ。

セレ： そうか。なるほどね。...でも、きっとすぐに決心(けっしん)が変わると思うな。三田くんはかわいい人(ひと)を見ると、すぐに好(す)きになっちゃうからね。

Sere: Don't worry. You're bound to find a good person one day.

Mita: It's all right. You don't have to try and make me feel better.

Sere: There's a match-making party next week. Why not try to go? I'm sure you'll have a good time.

Mita: No, I'm sure it won't turn out well again.

Sere: Huh? I've never seen you this pessimistic before.

Mita: I read in an article on the internet that in the near future, 1 in 4 Japanese men will be lifelong singles. I think that I'll be one of them, so I'm going to change my way of thinking so that I can live happily even if I stay single.

Sere: I see. Right ... but I'm sure that you'll change your mind soon. When you see a pretty woman, you'll fall for her on the spot.