～ことがある

ことがある has two meanings, and the verb form in front decides which. After a た-form it says you have done something at least once: "have ever", "have been". After the dictionary form it says something happens now and then: "there are times when".

The polite form is ことがあります; among friends it is ことがある.

Experience: the た-form

For "never", the verb stays in the た-form and ある becomes ない or ありません. が often turns into は in the negative.

• × 弾かなかったことがありません → 弾いたことがありません

An experience is something from further back. For what you did yesterday or do every day, use the plain past: ×昨日寿司を食べたことがある.

Now and then: the dictionary form

With the dictionary form the sentence describes how things go, not your past. Swapping the forms changes the meaning.

• × 日本に行くことがあります ("I sometimes go to Japan") → 日本に行ったことがあります ("I have been to Japan")