Setting up a mixed debian system?

Question

linux

There is debian (well, that is, a distribution kit with apt package management system). /etc/apt/sources.list contains sources for stable, testing, unstable and experimental. the "APT::Default-Release" option is stable.
I would like, while staying on a stable system, to install some packages from the test branch with dependencies on other packages from the test branch.
I specify in /etc/apt/preferences preferences for test versions of the necessary packages, but I can't install them due to dependencies.
Those. I wonder if there is a way to tell apt to install a given package and satisfy all of its dependencies and all of its dependencies, etc. from the desired repository.
At the time of installing the necessary packages, I tried to put "APT:: Default-Release" in testing, but this did not justify itself.
PS the problem is not to install the software, but to learn how to use the package administration tools apt / aptitude / dpkg / dselect, i.e. to collect from source codes / take other software is not an option yet.
ZZY debian-faq and apt help read - did not help :(

Answer 1

How are you trying to install a version from another release, and what exactly is wrong with the dependencies?
Usually, if you use something like (default release - stable)

aptitude install package -t testing

aptitude offers to update all required dependencies. in apt-get, the -t option also serves this purpose.

Answer 2

missed