Could vs. Was Able to

Attribute	Could	Was Able to
Ability	Expresses general ability or possibility	Expresses specific ability or achievement
Usage	Commonly used in hypothetical situations	Commonly used in past tense to describe completed actions
Formality	Considered more formal in some contexts	Considered more casual in some contexts

Definition

Could and was able to are both used to indicate past ability or permission. Could is the past tense of can, while was able to is the past tense of am able to. Both can be used to talk about past events or situations where someone had the ability to do something.

Usage

Could is often used to talk about general abilities or possibilities in the past. For example, "I could swim when I was a child." On the other hand, was able to is used to talk about specific instances where someone managed to do something. For example, "I was able to finish the project on time."

Modal Verb vs. Verb Phrase

Could is a modal verb, which means it is followed by a base verb. For example, "I could see the movie." Was able to, on the other hand, is a verb phrase that consists of the past form of "to be" (was) and the infinitive form of "to be able to." For example, "I was able to see the movie."

Formality

Could is generally considered more formal than was able to. In formal writing or speech, could is often preferred. However, in informal situations, was able to is more commonly used. For example, in a job interview, you might say "I could handle difficult situations" to sound more professional. But in a casual conversation with friends, you might say "I was able to fix my car by myself."

Negation

When negating could, we use "could not" or "couldn't." For example, "I couldn't find my keys." When negating was able to, we use "was not able to" or "wasn't able to." For example, "I wasn't able to attend the meeting." Both forms are used to indicate the inability to do something in the past.

Emphasis

Could is often used to express possibility or potential in the past. For example, "I could have gone to the party, but I chose not to." Was able to, on the other hand, is used to emphasize the accomplishment of a specific task. For example, "I was able to solve the problem after hours of work."

Conclusion

In conclusion, could and was able to are both used to talk about past abilities or permissions. Could is more general and formal, while was able to is more specific and informal. Both forms have their own unique uses and nuances, so it's important to understand when to use each one correctly in different contexts.