Mark can't/can play tennis. He plays on Tuesdays. Sara can’t/can play the piano. She's a good musician.
The baby can't/can walk yet.​

"Mark can play tennis." - This means that Mark is able to play tennis. He has the necessary skills and abilities to participate in this sport. The word "can" indicates that he has the capability to do it.

"Mark can't play tennis." - This means that Mark is not able to play tennis. He may lack the skills, physical ability, or opportunity to participate. The word "can't" indicates that he is unable to do it.

"He plays on Tuesdays." - This sentence tells us when Mark plays tennis. Specifically, he plays on Tuesdays. It could be a regular schedule for him, indicating that he practices or has matches on that day.

"Sara can play the piano." - This means that Sara has the ability to play the piano. She possesses the necessary skills and talent to play this musical instrument. The word "can" indicates her capability to do so.

"Sara can't play the piano." - This means that Sara is not able to play the piano. She may lack the skills, access to a piano, or interest in playing. The word "can't" indicates her inability to do it.

"She's a good musician." - This sentence tells us about Sara's musical abilities. It suggests that, despite not being able to play the piano, she still possesses the qualities of a good musician. This could mean that she is skilled at other instruments, has strong vocal abilities, or possesses a deep understanding and appreciation of music.

"The baby can't walk yet." - This means that the baby is not yet able to walk. Babies typically start walking at different ages, and in this case, the baby has not reached that milestone yet. The word "can't" indicates its current inability to walk.

In summary, the sentences are describing different people's abilities in different activities. The use of "can" and "can't" indicates whether they have the capability or lack thereof. The additional information provided in each sentence helps to give more context and understanding of the situation.