user36790, please don't take this answer the wrong way. It is not meant to be disparaging. Per your user page, you are 17 years old. You have some misunderstandings. You're way ahead of your peers, many of whom will hold similar (or even stronger) misunderstandings throughout their lives. You have asked a number of related questions over the last few hours. They all result from the same misunderstanding. This misunderstanding is that you are looking at things from a Newtonian point of view, where space is Euclidean, where time is an independent parameter, and where everyone agrees one what space and time are.
That's not how things work. It is very close to how things work under some special circumstances. Those special circumstances in which space and time locally appear to be distinct and Newtonian -- that's what we ordinarily experience on an everyday basis. This is why Newtonian mechanics has been so successful. That Newtonian mechanics works so well in our ordinary, everyday world does not mean that it is universally correct. In fact, we know it's not universally correct.
Then all paths must be curved. If so, how can there be any straight line motion?
This is your Newtonian mindset at work. Both special relativity and general relativity are markedly non-Euclidean. The sharp distinction between space and time in Newtonian mechanics becomes blurred in relativity theory; space and time become different aspects of one thing, spacetime.
Even though geometry in relativity theory is not Euclidean, one can still ask from the perspective of the non-Euclidean geometry of general relativity, "What is straight?" One definition of "straightness" in Euclidean geometry is that a straight line between two points is the path that has the shortest length amongst all the paths that connect the two points in question.
This concept of "straightness" extends nicely into the geometry of general relativity. All we need is something to measure "distance," a "metric," and this is something that general relativity provides. This generalization of a Euclidean straight line to a non-Euclidean geometry is called a "geodesic."