ことがある
- 1Expresses that an action, event, or state happens occasionally, not regularly.Ex. 1
- 2Expresses that the speaker or subject has had a certain experience at some point in the past.Ex. 2
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A neutral pattern used in both speech and writing to describe occasional habits, events, or states, or to talk about life experiences. The form before こと decides the meaning: dictionary form, ない-form, adjectives, and nouns give 'sometimes', while the た-form gives 'have done'. が is often replaced by も, as in こともある, to add the nuance that something 'also' sometimes happens.
Do not use the dictionary form when you mean past experience; 日本に行くことがある means 'I sometimes go to Japan', not 'I have been'.
Learners sometimes use the experience sense for ordinary everyday actions; it sounds odd unless the experience is notable.
Note that た-form + ことがない means 'have never done', while dictionary form + ことがない means 'never happens'.
- たことがあるN5「たことがある」 is simply this pattern attached to the た-form of a verb, meaning only "have done ~ before," with the negative 「たことがない」; this pattern, when attached to the 辞書形, ない-form, adjectives, or nouns, means "sometimes happens," and cannot be replaced with it.
- がちN3「がち」 is close only to the "sometimes happens" sense of 「ことがある」, but it indicates a high frequency that has become a tendency, usually with a negative evaluation; 「ことがある」 objectively states occasional occurrences. 「がち」 attaches to the ます-stem (ます-form minus ます) or to nouns, so the two cannot be interchanged.
- たびにN3「たびに」 means "every time ~, always ~," emphasizing that something happens on every occasion, which differs from the "sometimes" frequency of 「ことがある」, so they cannot be interchanged; 「たびに」 does not attach to the た-form and has no "have done before" meaning.
- かもしれないN4「かもしれない」 expresses a guess that something might be the case, with the speaker not certain; 「ことがある」, in its "sometimes" sense, states a situation that actually exists and occurs occasionally, and in its experience sense, states that something really did happen in the past. The two cannot be interchanged.
たことがある is the experience sense of this pattern specifically, while dictionary form + ことがある marks occasional occurrence; がち and 〜がちだ describe a frequent, usually undesirable tendency, and たびに means 'every time', whereas ことがある only says something happens sometimes. かもしれない expresses uncertain possibility ('might'), not something that actually happens on occasion.
