ことがある・
Something happens now and then: "sometimes", "there are times when". Watch the verb before こと. In the た-form, the same words mean "have done before".
sometimes do
Once in a while I walk the length of one stop on my way home.
sometimes don't
On busy mornings I sometimes skip breakfast.
How to form it
Verb in the dictionary form or the ない-form + ことがある／こともある
行くことがある, 行かないことがある. Polite ことがあります. An adjective can come before it too: 寒いことがある.
食べることがある or 食べたことがある?
I sometimes eat lunch at this restaurant.
Dictionary form: it happens now and then.
I have eaten lunch at this restaurant before.
た-form: it has happened at least once.
Now and then
It is for what happens some of the time, so たまに (once in a while), ときどき (sometimes) and まれに (rarely) go well with it. For something that happens often, people say ことが多い, or just the plain verb with よく.
Even now, my grandmother sometimes hums old songs.
When I'm tired, I sometimes don't wake up even when the alarm goes off.
こともある is "there are also times when". You have the usual case in mind, and this is the other one.
I usually eat at home, but once in a while I eat out.
ことがある or ことが多い?
ことが多い is "often": 多い, "many", stands where ある was, so the times are many. The habit can be good or bad.
On weekends I sometimes watch a movie at home.
Now and then.
On weekends I often watch a movie at home.
Often.
よく says "often" only before a verb that isn't negative: よく見る. With a negative or an adjective it is ことが多い: before a negative, よく means "well", as in よく眠れない, "can't sleep well".
On weekdays I often skip breakfast.
On notices and labels: "may"
Station notices, medicine labels and out-of-office replies use it to warn you that something can happen. English says "may".
While I am away on business, my replies may be delayed.
In rare cases this medicine can make you very drowsy.
That "may" still means "it happens now and then". A guess about what will happen this time is かもしれない.
In winter the trains are sometimes delayed by snow.
It happens now and then.
The trains may be delayed by snow tomorrow.
A guess about tomorrow.
Close neighbours
ことはないN3
It looks like the negative of this one, but said to a person it is "there's no need to".
The mistake can be fixed, so there's no need to feel so down.
Check yourself
1What does this mean?
兄は
Choose an answer to see why.
2"I might be late for tomorrow's meeting."
Choose an answer to see why.
More detail
There were times when
About the past, the end changes. The verb before こと stays as it is.
When I was a student, there were times I studied until morning.
Never
The negative is ことがない or ことはない: it never happens.
My father never talks about work at home.
More sentences
Even on days when I work from home, I sometimes go into the office just for the afternoon.
In this area, it sometimes snows even in spring.
Sometimes the train is late and I don't make it to the meeting in time.
Mountain weather can change suddenly in the afternoon.
In online meetings, the sound sometimes cuts out for a moment.