For example, "can" lacks an infinitive, future tense, participle, imperative, and gerund. The missing parts of speech are instead supplied by using the appropriate forms of "to be" plus "able to". So, while "I could write" and "I was able to write" have the same meaning, "I could" has two meanings depending on use, which are "I was able to" or "I would be able to". One cannot say *"I will can", which is instead expressed as "I will be able to". Similarly, "must" has no true past tense form, this instead being supplied by "had" (the past tense of have), and "to have to" in the infinitive, an example of composite conjugation. The past tense expressing the obligatory aspect of must is expressed as "had to", as in "He had to go." "Must have", on the other hand, expresses probability or likelihood in modern English, e.g., ""If that's thunder, there must have been lightning.""