Form 8843 is a Statement for Exempt Individuals who use to show they are not counting their days within the US. Form 8833 is a form you file to claim tax treaty benefit.
In your situation, you might not need the Form 8843. On the 6th year on your F visa, you have already passed the five years exempt period. Starting from January 1st, 2019, if you stay more than 183 days within the US, you will be a resident. You do not file the Form 8843. If you do not stay more than 183 days, you will still be a nonresident then you need to file Form 8843 to show your exempt status.
As far as the Form 8833, you only need to file this form if there is a tax treaty applies to you and you need to file to show the reduction of your income.
For people with OPT or H-1B visa, this happens usually on the 6th year of the F-visa ( not sure if this applies to you), you switch to H-1B later in the year. In this case, you do not need Form 8843 as most likely you have stayed more than 183 days in the year ( while on the F and H-visa) hence you are a resident. You only need to file Form 8833 if there is a treaty benefit.
