Careful. This reasoning is not foolproof. Take the following sentence for example:
Me alegro de que hayas llegado.
I am happy you have come.
There is no doubt or uncertainty the the person has come. He is standing right there. The reason for subjunctive is twofold:
The sentence is focused on your happiness, the subjunctive cause explains why you are saying you are happy, it is considered the minor player in the sentence.
There is no reason for you to "declare a new fact or belief" by using the indicative. The person is standing right there. Here knows he is there. Everyone else within earshot knows he is there. When everybody already knows something, that part of the sentence usually goes in the subjunctive, while the "new information" "I am happy" goes in the indicative.
creer que doesn't express certainty. It expresses belief, thought or opinion. In Spanish, when you say what you think or believe, you normally say it using the indicative.
If you say that MAYBE (tal vez, quizas) you think something you can say it using either the indicative or subjunctive.
If you say something by using a subordinate clause with que, for example: Es probable que .... Then you have to use the subjunctive.
The idea of "certainty or doubt" or reality or irreality can be helpful, but they are not conclusive guidelines. You have to learn the various phrases and learn which phrases allow which mood, and under which circumstances.
Mstahr, Legend's comment at the top of this page explains it, but here is some more detail. "Creo que" in Spanish does not express doubt, it is an assertion, an affirmation, and therefore takes the indicative mood. Creo que él es muy simpático. "No creo que" triggers the subjunctive. No creo que él sea muy simpático. Pensar and suponer work the same way.
Was thinking is incorrect English to expressed the simple idea that she thought something. Only if was to express an active thought process, for example, I was thinking about where to go on vacation, is is proper English. There was no "active thinking process occurring" in this example. It was just something that she believed. The imperfect is used because she probably believed it prior to this statement being make and afterwards. We are not making any comment, in Spanish, by using the imperfect about how long she believed this, or even if she STILL believes it. For this reason the "used to believe" or "used to think" option should not be accepted in this case because it implies that she no longer believes that we are friends which the Spanish imperfect does not say at all.
Some of the translations reflect 'used to'. However, I understood the 'aba'/'ia' suffixes to relate to an action in the past that may not be resolved, ie. was or were doing something. So, while it is correct to say 'used to believe', it could also be correct to say 'was believing' in that the action had begun in the past, but had not been resolved or concluded. (hablaba = was talking, hacia = was doing, etc.)
I think when DL started they made a distinction between "creer"="believe" and "pensar"="think" but over time the database has come to accept, as it should, that "think" and "believe" are often interchangeable in English (Nb: The same is not so true of "creer" and "pensar" in Spanish).
Good question. It highlights how our different versions of representing the Spanish imperfect in English work. I don't think your sentence is wrong, it just sounds odd, because we wouldn't normally double up the "used to."
All the imperfect is saying is that something happened over a time instead of at a time. As her holding this belief wasn't a momentary event we need to use the imperfect in Spanish. We can translate this as "She used to believe" or "She believed" but DL has a tendency to use the "used to" structure because it removes ambiguity ("She believed" would also be a valid translation for the Spanish preterite).
Personally I dislike the "used to" construction as I think it conveys an over and done with concept that isn't absolutely necessary, but I can understand why DL and many other platforms choose to use it to represent the imperfect.
Anyway, now we can look at what she used to believe: that we were / used to be friends. Again this is something that happened over a period of time, it wasn't a single event, so we need the imperfect, but now we can safely choose "were" as opposed to "used to be" because it is more natural English.
It is generally fine to use the "used to" phrase for repeated or habitual ACTIONS in the past. But with "state / status" verbs that merely describe what someone thought, believed, loved, had, etc. in the past, the "used to" phrase every time the Spanish imperfect is used is just plain wrong.
The "used to" phrase should only be used for repetitive ACTIONS although duo also tends to accept it, erroneously, for "status verbs" such as to think, to believe or to have. When used for these verbs, using "used to" implies that this status is no longer true. The Spanish imperfect is silent regarding whether something that was true in the past continued to be true after the point referenced or even whether or not it is true today.
RABayer, I hope I can help! (Although noting that this was two years ago, you probably have solved this already.)
In the present tense of the English verb "to be", "we are" contracts to "we're". So you are right that "we are" and "we're" are synonymous.
On the other hand, you may be thinking that "we were" can be contracted. It cannot. The past imperfect conjugations of "to be" do not contract with the subject pronouns, not for "we" or any other subject pronoun*.
So, since this Spanish sentence is in the imperfect past, "she used to believe we were friends", what she was believing would also be in the past. Past imperfect or "to be": I was, you were, he/she/it was; we were, you were, they were.
*The only exception to the no imperfect contractions comment is the third person singular "it" + was". Thus, --> 'Twas the night before Christmas.... --> 'Twas brillig and the slithy toves did gyre and gimble.... Note that the vowel "i" gets dropped in the " 'Twas" contraction.
My professor explained the use of subjunctive like this: You use the subjunctive in four different aspects, those being obligation, negation, causation, and emotion/doubt. Remeber the mnemonic ONCE (obligación, negación, causación, emoción), and you'll never go wrong. :)