Grammatically, I agree with Pjt: The agreement with the verb in preterite can only take a past subjunctive.
Yet, there are reasons why the "companion" of "advirtió" may be something else than "tuviera
*":
1.- When the speaker has in mind the
situation that has all happened in the past. When the police warned of some risk to the people, and the risk is over now, then "
tuviera" must be the only choice.
But if the risk is still present now, the speaker's choice will likely be "
tenga":
· La policía advirtió a la gente que tenga cuidado.
(People now must still take care of themseleves due to the fact that the risk isn't over yet.)
2.- Many people in South America (and lately, due to the influence of social networks in many other places), have ditched the past subjunctive and use the present subjunctive instead. So, they'll use "
tenga",
even if the risk is over and the situation has all happened in the past.· La policía advirtió a la gente que tenga cuidado.
(People were in danger and were warned by the police, all in the past, but now the risk is over. However, for reasons of colloquial usage, the speaker is using "tenga" instead of "tuviera".)
3.- The companion of the verb may be an infinitive ("
tener"):
· La policía advirtió a la gente tener cuidado.
This is hard to explain, so I'll do it by parts:
In grammar manuals, the rule is that if the same person performs two actions, then the first verb is conjugated and the second one is an infinitive. I was taught this is mostly a matter of style rather than a rule, because in colloquial usage, many sentences with the same subject have a subjunctive and a few sentences with different subjects take an infinitive; so I will try to explain as I learnt it.
Consider these sentences:
· Vine hoy para que (yo) no tenga que venir mañana.
· Vine hoy para no tener que venir mañana.
(The same person is the one who comes today, and expects not having to come back tomorrow, so the infinitive provides a shorter, more smooth and natural sentence.)
· Lamento que (yo) te haya hecho sentir mal.
· Lamento haberte hecho sentir mal.
(The same person made the other feel bad and the same person regrets having done it. The infinitive doesn't only avoid the longer sentence, but also clears up the possible ambiguity about a third person having made feel bad the one I'm talking to.)
· Esperamos que (nosotros) no hayamos llegado tarde.
· Esperamos no haber llegado tarde.
(The second sentence is more elegant and flows much better than the one with the subjunctive.)
All of these sentences have the same subject performing two actions and the ones with the infinitive are far better than the ones with the subjunctive, because they're shorter, and offer no ambiguity in meaning.
As for the sentences where there is more than one subject performing different actions, the normal formula is like the one in your example:
· No creo que Juan sepa de coches.
· Espero que Olivia me llame.
· Roberto me dijo que viniera. (If the situation is all in the past.) / Roberto me dijo que venga. (If I don't consider the situation is completely over yet.)
In all these cases, the
first subject and the second one have a
clear first action and a second action they're performing, so there is no confusion about whether to use an infinitive.
Normally, when there are two subjects, but there is an infinitive instead of a subjunctive, it's easy to identify that both sentences have
different meanings:
· Los niños querían que abriera los regalos.
· Los niños querían abrir los regalos.
(In the first sentence, the children are insisting that I open the presents, but in the second one they want to open them, either all themselves or we all together.)
· Nos llamaron para que compráramos pan.
· Nos llamaron para comprar pan.
(In the first sentence, they're calling for us to buy bread, and in the second they're calling to buy the bread from us.)
So now, for the example: "La policía advirtió a la gente tener cuidado."
Here the meaning of the sentence with an infinitive is clearly the same as when using a subjunctive, because the subject is tacit in the infinitive. When I think about the examples, it seems to me this is commonly done when the sentence implies some
instruction, order or recommendation:
· Es necesario que le digamos que se vaya.
· Es necesario decirle que se vaya.
(Here, we're using an impersonal as first subject and a "nosotros" to talk about the group of people who wants a third person to leave. In this case, the impersonal and the infinitive is a better agreement than than an impersonal and "nosotros", but since we are the ones who want this person to go away, "nosotros" is implicit in "decirle".)
· El doctor ordenó que (ella/él) se tomara/tome la medicina. ("tomara" if I want to agree with the preterite, "tome" if I feel they still have to take the medicine.)
· El doctor ordenó tomarse la medicina.
(The first sentence marks clearly both subjects and both actions, which is grammatically better. The second sentence makes the infinitive about one person that we know is sick.)
· Le sugerimos que no conteste (usted) llamadas de desconocidos.
· Le sugerimos no contestar llamadas de desconocidos.
(In both sentences there is one person who must not answer calls from strangers, just in the first one is explicit and in the other one the infinitive avoids giving a direct instruction to this person to be more polite.)
So that's why it can't only be "tuviera", although grammatically, it's the best choice.
If you have questions, let me know. You might also find useful the discussions
here and
here.
*I'm using "tuviera" instead of "tuvieran" because I prefer the agreement with "gente", which is a singular noun.
