When you want to express that something will probably happen, but are not certain, you can use the word 多ta分bun.
|(He)'s probably already sleeping.||多分 もう 寝て いる。tabun mou nete iru.|
|The book is probably in the library.||本 は 多分 図書館 に あります。hon wa tabun toshokan ni arimasu.|
A more formal word than 多ta分bun is おそらく. It is equivalent to 多ta分bun in its level of certainty, namely the thing it described is felt as probable but not certain.
|That is likely not the correct answer.||おそらく それ は 正しい 答え で は ありません。osoraku sore wa tadashii kotae de wa arimasen.|
|The shop is likely closed.||お店 は おそらく 閉まって います。omise wa osoraku shimatte imasu.|
When you have a higher degree of certainty, you can use きっと, which roughly translates to surely or definitely, and indicates a strong belief or expectation.
Another word that translates to surely or definitely is 確か に. When using 確か に, the speaker indicates that their belief is based on their knowledge, experience, or evidence. So while both words can be translated to certainly in English, きっと is more about expressing a strong expectation or belief, while 確かに is more about confirming or asserting a known fact. 確か に is also often translated as indeed.
|That shop is certainly closed.||確か に あの 店 は 閉まって いる。tashika ni ano mise wa shimatte iru.|
|This is indeed his wallet.||確か に これ は 彼 の 財布 です。tashika ni kore wa kare no saifu desu.|
Finally, the word 必ず can be used to express a very high level of certainty. It is equivalent to definitely or without fail.