When Aristotle says substance in the truest and primary and most definite of the word, is that "which is neither predicable of a subject nor presents in a subject", he is saying that physical things are a substance and there would always be something to describe them in other words a predicate from a subject but substances themselves cannot be predicates with other substances. Primary substances can have predicable and present things for example an apple can have a predicate "apple" and something is present in it, the colors. Aristotle talked about secondary substances which were characteristics of primary substances. The secondary substances need the primary ones to exist and some of them are unique identities rather than describing the primary ones.
Another thing that he believed was that if primary substances did not exist, nothing else would exist when he says.and if this last (primary substances) did not exist, it would be impossible for anything else to exist. Considering his point of view this would be true because taking away the characteristics and qualities from something it would not really have a purpose in life. In other words if we go back to the example of the apple, taking away the characteristics and the qualities of the apple which in this case would be the color, the form and the and the flavor of the apple the it would be no point for that apple to exist.
Primary substances can change while staying the same was something else he thought when he says, the most distinctive mark of substance appears to be that one remaining numerically one the same, it is capable of admitting contrary qualities. This tell us that things change and not specifically just by forms but also on the inside just like a person that is good and can be bad too while being the same human being. Aristotle's ideas, thinking about them today were some kind of common sense it's just that nobody really thought about them then deeply as he did, it all starts from primary substances and after that all predicates can be applied, secondary substances can exist and qualities can be constantly changing.
