When Q increase from 1 to 2 TR increase from 10+18=28. MR = 18−10=8. Noticed that Q is increase, Price = AR is falls and MR falls

Marginal Revenue (MR) is the change in total revenue ($\Delta TR$) divided by the change in quantity ($\Delta Q$).
Given:
When $Q$ increases from 1 to 2, $TR$ increases from 10 to 18.

$$\Delta TR = TR_2 - TR_1 = 18 - 10 = 8$$ $$\Delta Q = Q_2 - Q_1 = 2 - 1 = 1$$ $$MR = \frac{\Delta TR}{\Delta Q} = \frac{8}{1} = 8$$

Your calculation $MR = 18 - 10 = 8$ is correct.

In markets where a firm has some market power (e.g., monopoly, monopolistic competition), to sell more quantity ($Q$ increases), the firm must lower its price. Since Average Revenue (AR) is equal to Price ($AR = \frac{TR}{Q} = P$), if the price falls, then AR also falls. When AR is falling, MR will always be less than AR and will also be falling. This is a fundamental relationship in such market structures.
Your observation that "when $Q$ increases, Price = AR falls and MR falls" is correct.

✅ Correct. All your calculations and observations are accurate based on the principles of marginal revenue and average revenue.