"I used to" and "I was used to" - What is the difference?

Question

Don't the two sentences:

1. I used to drink coffee every day.
2. I was used to drinking coffee every day.

refer to a past habit? If not, what is the difference?

Answer 1

Yes, they both refer to a past habit, but the meanings are different. "Used to" is used in two ways.

I used to drink coffee every day.

This just recounts what you drank and how often you drank it, and implies that this was in the past and you don't do it any more. This usage of "used to" means that something existed or repeatedly happened in the past but does not exist or happen now.

I was used to drinking coffee every day.

This describes how your body handled drinking coffee every day. The meaning could be "I used to drink coffee every day and I was accustomed to it, so it didn't make me feel agitated. But if I did that now, I would never sleep at night.

This usage of "used to" refers to being familiar with something so that it seems normal or usual.

See M-W.

Answer 2

Your two sentence are slightly different in meaning, one is about drinking coffee, the other about the habit of drinking coffee, the habit aspect is implicit in the your first sentence and explicit in the second

I used to drink coffee every day.
habitually I drank coffee every day

I was used to drinking coffee every day.
I had become accustomed to drinking coffee every day.

Answer 3

The phrase "used to" functions more or less like a modal, similarly to "did" (though with exceptional behaviour when there is do-support).

I used to drink coffee every day.
→ I formerly drank coffee every day.

The phrase "be used to" is an entirely different animal. Here "used" has become an adjective.

I am used to drinking coffee every day.
→ I am accustomed to drinking coffee every day.

I was used to drinking coffee every day.
→ I was accustomed to drinking coffee every day.

There's no particular connection between the usage of these words, despite their apparent similarity.