Can you help me answer some questions about using the word “The”?

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I. In the following sentences, I know that I should repeat "they", "they", "she", "he" and "it" instead of using "the " in the second part. But can I use 'the + noun' if I introduce something using the verb "be" then talk about it with the second sentence directly following the first, not later on, and can the hearer understand that "the + noun" mean "the noun I called it earlier"? Ex:

1 This is a letter. The letter is written in spanish. (Does "The letter" mean "the letter I called it earlier"?)

2 It is a box. The box is red. I will open the box and … (Does "The box" mean "the box I called it earlier"?)

3 That is a thief. The thief is tied to a post by a rope. (Does "The thief" mean "the thief I called they earlier"?)

4 They are doctors but the doctors are not kind. (Does "The doctors" mean "the doctors I called they earlier"?)

5 They are slaves, and the slaves are mine. (Does "The slaves" mean "the slaves I called they earlier"?)

6 She is a slave, and the slave is mine. (Does "The slave" mean "the slave I called they earlier"?)

7 He is a doctor but the doctor is not kind. (Does "The doctor" mean "the doctor I called they earlier"?)

II. And when I use the other verbs, not the verb "be", so are they grammatically correct and natural? Ex:

8 I got a present. The present is a cat. I will give John the cat.

9 I bought a ball yesterday. The ball is red. When I kicked the ball, the ball flew over the fence.

10 She is biting an apple. The apple seems very rigid.