Oxford Advanced Leaner's Dictionary says:
You use "was/were able to" or "manage" (but not "could") when you are saying that something was possible on a particular occasion in the past.
However, I often hear the following conversation.
"Thank you for inviting me over."
"I'm glad you could come."
May I understand that "I'm glad you could come" is acceptable because it is an idiomatic expression?
Is it strange to use "I'm glad you were able to come" in the above case?
What about the following sentence? Must I use "was able to" instead of "could"?
"I ran fast and I could catch the bus."
Could expresses an ability that one no longer has:
Ex: She could run fast in those days (i.e., had the ability then but now no longer does).It is in the above sense that the meaning of could is deemed awkward in, say,
Ex: This morning, she could catch the bus because she ran fast.
The correct usage:
Ex: This morning, she was able to/managed to catch the bus because she ran fast.Ex: In those days, she could catch the bus because she ran fast.The example I'm glad (that) you could come doesn't fit the rule. Modal could is used as a polite expression and for that reason is synonymous with were able to; e.g., I'm glad (that) you were able to come.