Answer:
There's something you must know.
Explanation:
All of the incorrect sentences contain a subject-verb agreement mistake. The subject-verb agreement refers to that the subject and verb must agree in number (singular or plural).
<em>There's pencils in the box</em> should be<em> </em><em>There are pencils</em><em> in the box. </em>As the subject is plural, the verb must be in the plural form as well (are + pencils).
<em>There's five marbles on the floor</em> should be <em>There are five marbles</em><em> on the floor</em>. Once again, there is more than one marble, so the verb has to be in the plural form (are + five marbles).
<em>There's several ways to post something online </em>should be corrected to <em>There are several ways</em><em> to post something online</em>. Just like <em>all, both, few, many, </em>and <em>some</em>,<em> several</em> goes together with the plural form of a verb, although in colloquial language you can often hear it used with the singular. <em>Are + several </em>is the grammatically correct option.
<em>There's something you must know</em> is an example of correct subject-verb agreement, as the singular form of the verb (is) is used with the singular pronoun <em>something</em>.