  The weakly-coupled heterotic string is known to have problems of
dilaton/moduli stabilization, supersymmetry breaking (by hidden-sector gaugino
condensation), gauge coupling unification (or the Newton's constant), QCD
axion, as well as cosmological problems. We study these problems by adopting
the viewpoint that they arise mostly due to our limited calculational power,
little knowledge of the vacuum structure, and an inappropriate treatment of
gaugino condensation. It turns out that these problems can be solved or are
much less severe after a more consistent and complete treatment. There are two
kinds of non-perturbative effects in the construction of effective field
theory: the field-theoretical non-perturbative effects of gaugino condensation
(with a constraint ignored in the past) and the stringy non-perturbative
effects conjectured by Shenker, which are best described using the linear
multiplet formalism. Stringy non-perturbative corrections to the K\"ahler
potential are invoked to stabilize the dilaton at a value compatible with a
weak coupling regime. Modular invariance is ensured through the Green-Schwarz
counterterm and string threshold corrections which, together with hidden matter
condensation, lead to moduli stabilization at the self-dual point where the
vev's of moduli's F-components vanish. In the vacuum, supersymmetry is broken
at a realistic scale with vanishing cosmological constant. As for soft
supersymmetry breaking, our model always leads to a dilaton-dominated scenario.
For the strong CP problem, the model-independent axion has the right properties
to be the QCD axion. Furthermore, there is a natural mass hierarchy between the
dilaton/moduli and the gravitino, which could solve the cosmological moduli
problem and the cosmological problem of the model-independent axion.
