The formal form -(으)ㅂ시다 you can use to suggest or propose to do something to others. However, this is a rather special form to which a few rules are attached to when it is suitable to be used. In general, this form is the one you will likely use the least of all forms to make suggestions and propositions.
The form -(으)ㅂ시다 is a formal form to suggest or propose to do something. However, while it is classified as a suggestion or proposition, in truth in most cases it is neither. It is a rather soft way to give an order. Therefore, using the form incorrectly can cause a few eyebrows to be lifted.
As mentioned, the formal form -(으)ㅂ시다 has a few important restrictions when it is appropriate to use the form. There are three main situations you need to keep in mind. For other situations, other forms, like -(으)ㄹ까요 or -(으)ㄹ래요, are more suitable.
가: 언제 공항에 출발할까요?
나: 20분 후에 출발합시다.
A: When shall we leave for the airport?
B: Let’s leave in twenty minutes.
How To Use
Using the form -(으)ㅂ시다 is not difficult at all. Perhaps you can tell, but its conjugation rules are based on the ever important ‘batchim/no batchim’ rule. This means you just have to look whether the verb stem ends in a consonant or not. If it ends in a consonant, you use -읍시다, if not, it is -ㅂ시다. Of course, the Korean irregular verbs can cause a few issues, especially the ㄹ irregular verbs.
Lastly, keep in mind that this form can only be used with verbs. So do not try to add it to any adjectives, because that would be incorrect.