Besides the past tense and the present tense, you also have the future tense. In Korean, the future tense has the form of -(으)ㄹ 거예요. However, you can also use this form to express a supposition. Suppositions that use this form are always based on objectives facts that you know.
The most important use of the form -(으)ㄹ 거예요 is its use as the future tense in Korean. Like in most languages the future tense, you use this form to talk about how things will be in the future or things you plan to do in the future. However, to express your future plans or intentions more strongly other forms are more suitable, such as -(으)ㄹ게요.
가: 내일 비가 올 어예요.
나: 그럼 제가 우산을 사야 해요.
A: Tomorrow it will rain.
B: Then I need to buy an umbrella.
Conjecture And Supposition
You can also use the form -(으)ㄹ 거예요 to express one of your suppositions. Whereas -(으)ㄴ/는 것 같다 implies the supposition is made without a solid basis, the supposition you make with -(으)ㄹ 거예요 is one you make based on facts you know. These facts are either based on things you have experienced, seen or experienced. Therefore, you are pretty certain your supposition to be true.
가: 민수 씨는 회사에 아직 있어요?
나: 7시니까 이미 퇴근할 거예요.
A: Is Minsu still at the company?
B: It is 7:00, so I think he has already left.
How To Use
The form -(으)ㄹ 거예요 is is rather simple to use. However, there are some differences between using it for expressing supposition and using it as the future tense. Though these differences are minute and easy to remember. When it comes to conjugation, the form simply follows the ‘batchim/no batchim’ rule, but you will have to keep the verbs with a ㄹ as batchim in mind. As you know those are treated as no batchim. Lastly, keep the irregular verbs in mind.
Now to grammatical differences between the uses. This is rather simple. You cannot use -(으)ㄹ 거예요 in a question when you use it to express your supposition. If you want to express a supposition in the form of a question, you use the form -(으)ㄹ까요. Of course, when you use the form for the future tense, you can use it in questions without a problem.