Tenía que or Tuve que?
I want to say: Last month, I had a very serious problem. I had to buy all of my clothes.
I'm quite sure the first sentence is: El mes pasado, tuve un problema muy grave.
But then I'm not sure about the next sentence because it was a problem that occurred over an extended period of time.. So is it "Tenía que comprar" or "Tuve que comprar"?
10 Answers
Tenía un grave problema. Tenía que comprar toda la ropa.
So is it "Tenía que comprar" or "Tuve que comprar"?
Which one do you prefer? If your book tells you that for every sentence there is only one option, throw it to the bin and get yourself a new one. Both can be used. The question is which one do you want to use? Each language requires that you consider many factors when you use them, and here Spanish allows two possibilities that any Spanish native considers every time one wants to talk about the past. English does not offer this distinction for a sentence like this (at least, I can't see one), so you have to start learning how a Spanish native thinks and try to do the same if you want to use these tenses appropriately. Since both sentences are translated the same into English, a translation does not help here, so learn the basic imperfect-preterite difference, and forget English.
But I specify the time period in the first sentence, so isn't it supposed to be preterite?
No. The problem happened that month, but haven't stated the precise boundaries of the event. Surely the problem did not start at 0:00 h at the beginning of the month, and finished as 23:59 h on the 30th, did it?
These are just suggestions, and any advice, comments or corrections are certainly more than welcome:
El mes pasado, tuve un problema muy grave. Tenía que comprar...
Last month, I had a big problem. I was having to buy...
Here the temporal reference is last month. That is, the fact of the speaker having a problem is viewed as completed (for example: Last month I had a problem, but this month is different). For the second sentence, the temporal reference is still last month, only now, because the imperfect is used, the action is anaphoric in that it relates directly to the preceding utterance. Moreover, the fact of the speaker having to buy his clothing makes no reference to any initial or final boundary to this event. Instead, it simply implies, without specifying temporal boundaries to the event, that for some unspecified time interval(s) during the previous month, he was having to buy his clothing.
El mes pasado, tuve un problema muy grave. Tuve que comprar...
Last month, I had a big problem. I had to buy...
Here, the first sentence is the same as (1) above; however, in the second sentence, the fact of the speaker having to buy his own clothing becomes a localized event, one in which the action is clearly delimited by the use of the preterit. That is, the preterit focuses on the completedness of the action.
El mes pasado, tenía un problema muy grave. Tenía que comprar...
Last month, I was having a big problem (I was in the midst of a big dilemma). I was having to buy...
Using the imperfect in both sentences here, allows the speaker to detail the events as if he were reliving them. That is, we find the speaker in the midst of the problems that he was having last month; namely, he was having to buy all of his clothing.
El mes pasado, tenía un problema muy grave. Tuve que comprar...
Last month, I was having a big problem (I was in the midst of a big dilemma). I had to buy...
Here, again the speaker describes an ongoing dilemma in the past (sometime last month) which sets the scene for an event that occurred (i.e. was initiated/completed) while this was going on. Again, the use of the preterit acts as a delimiting factor so that the fact of the speaker having to buy his own clothing is viewed as a completed action with reference to some temporal reference point (i.e. an action completed at a specific point in time).
I am sorry but I am confused by the answers , is it "preterite" or " imperfect" ? 
Spanish allows two possibilities that any Spanish native considers every time one wants to talk about the past. English does not offer this distinction for a sentence like this (at least, I can't see one) Lazarus
Just as a side point Laz, although 'I had to' works as an equivalent for tuve and tenía que we would also have the option in English of saying 'I was going to have to' if we wanted to stress the futurity of it.
Ok here's my take just to keep it simple:
tuve que .... preterite = completed action
tuve que ir a la oficinia - I had to go (and did) to the office.
tenía que ... imperfect = a developing action
tenía que ir a la oficina - I had to go to the office - I knew it but no action is implied here it's just a mental state.
Hope that's right and it helps someone 
the majority of learners stick to preterite for their statements
I'm just a beginner and I don't do that. I use the imperfect for situations and the preterite for events.
I always try to remember it as "used to". For example, tenía que comprar" I would think in my head as "I used to have to buy", but I don't anymore.
And tuve que comprar as "I had to buy", as more of a sudden thing that I just did.
The imperfect and preterite differences are extremely hard for most language learners, especially given that English doesn't really specify the use of either. If we used the imperfect, like "I used to eat" there is the implication that it doesn't happen anymore, but if we were to use the preterite/past e.g "I ate.." there is the implication that the action finished at that point.
As I said, it is confusing, and don't be disheartened if you don't understand it straight away. Being an advanced Spanish learned (8 years or so), I still have trouble using them, and in the classes where I study, the majority of learners stick to preterite for their statements, UNLESS it is actually "The government used to ignore the problem.. but now they recognise it", for example.
I'm sure if you bought a grammar book and practised some of the exercises, or just did some games on the internet, it would help you to grasp the differences. Then again, context is everything - usually you can do it from the sentence.
Hasta luego
If you are describing your need as a completed event in the past, you would use the preterite. If you are describing your need as a situation (in which something happened), you would use the imperfect. If I still needed to do something, I would use the present tense although the imperfect could also be used.
It has more to do with what you are saying than what happened.
It all depends on the context of the sentence. For example in the second part if you said "Tuve que comprar.." it would mean "I had to buy" (It was necessary and I did) ; while if you said "Tenía que comprar.." It would mean "I had (was supposed) to buy."
