This is not an English sentence, and why does the second verb take a subjunctive rather than an infinitive form?

Hola Allison

Sentences introduced by an opinion in "negative" are followed by the subjunctive. Have a look at this lesson that will clarify your doubt:

[competency id= "6184"]

The English typo has been corrected. 

Muchas gracias,

Inma

Hi Allison

That is correct in certain cases, for example:

"Yo quiero estudiar más" (same subject in both, infinitive) but 

"Yo quiero que (tú) estudies más" (different subjects, needs subjunctive)

Also with "hope":

"Yo espero ganar la lotería" (same subject in both, infinitive) but

"Yo espero que ganes la lotería" (different subjects, needs subjunctive)

Notice the absence of "que" in the sentences with the same subject.

In order to be able to use an infinitive with "Yo no creo que..." you need to get rid of "que". For example:

Yo no creo haber hecho todos los deberes para el lunes. (although this sounds a bit formal)

With "que", the only option would be:

Yo no creo que haya hecho todos los deberes para el lunes.

Saludos

Inma

Hi Allison

Sorry about this big delay on replying to your last query. The notification didn't come through back in October. 

So, yes, this is an option too but it would sound very formal:

"Yo no creo haber hecho/terminado todos los deberes para el lunes."

Un saludo

Inma