事(こと)があるSomething is possible, Sometimes, Occasionally, There are times when

The difference between ことはない and ことがある in Japanese lies in their suggested frequency and necessity. The former, ことはない, indicates that there is no necessity for a certain action to happen, or it never happens due to its low frequency or irrelevance. On the other hand, the latter phrase, ことがある, denotes the possibility of an event's occurrence, suggesting that it happens sometimes or occasionally.

The grammar point ことがある is used to express the idea that something happens occasionally or there are times when something occurs. It is often used to express an event that is not done regularly but happens sometimes. On the other hand, たことがある is used to express past experiences and is used to say that one has had the experience of doing something in the past. It reflects an action that the speaker has done at least once before.

The grammar point "ことがある" is used when referring to occasional or possible experiences in the past or future, indicating that there are times when certain actions are done or certain situations occur. On the other hand, "とき" is used to state the time when a certain condition or event takes place, either in the past, present, or future. Although both expressions can be used to refer to occasions or times, "ことがある" emphasizes occasional instances, while "とき" simply clarifies the timing of an event.