How do I know when gravity is postive or negative?

Gravity is always going to pull you downward.  The direction of the y axis is arbitrary and the sign of the downward pull is negative if the y axis points up, but the sign of the downward pull would be positive if the y axis pointed down.

Definition: The acceleration due to gravity is a constant 9.81m/s² downwards.
Because acceleration is a vector, it is necessary to specify it with a magnitude (9.8m/s²) and a direction (downwards) ( note that the magnitude value does not carry any sign, and you cannot interpret this as being positive. 9.81m/s² is a magnitude only )

In physics, the direction  upwards is usually taken as being positive. In doing any problems, and I teach my students this, always specify this in your answer and then there is no confusion.
Because you have made this decision on sign and direction, UP = +, Then,acceleration due to gravity (down) must have a negative value.
In your question you state: and I paste directly: the acceleration of gravity is constant,  DOWNWARD, and has a magnitude to g=9.81m/s^2.  You have assumed that the definition including the magnitude 9.81m/s² is positive. Nowhere in this definition from your textbook does the magnitude of the acceleration carry a positive sign.
The above is a well used convention and it is then common for acceleration down due to gravity to be negative, Remember that a negative acceleration , such as g= -9.81m/s² does not mean that the object is slowing down. Unfortunately many students, even in this forum think this.
If you throw a ball into the air, and choose up as positive, then the ball will travel upwards accelerating at -9.81m/s²  until it reaches the highest point of its travel, and then it will fall back to earth, once again accelerating at -9.81m/s²

You are totally free to ignore this convention and choose DOWN as positive, and under these circumstances, the acceleration due to gravity will automatically have a + value. But you must state your decision on direction and sign at the beginning of your answer and then neither you nor your examiner will be confused.

After this long discourse I reread your question: I hope that you can see that you have made the wrong assumption when you say:

Why did gravity all of a sudden become negative when the first part says gravity has a set downward motion to be positive 9.8m/s^2??

Nowhere has the first part said that the acceleration is positive 9.8m/s².