  We study the properties of pions in twisted mass lattice QCD (with two
degenerate flavors) using chiral perturbation theory (ChPT). We work to
next-to-leading order (NLO) in a power counting scheme in which m_q ~ a
\Lambda_QCD^2, with m_q the physical quark mass and a the lattice spacing. We
argue that automatic O(a) improvement of physical quantities at maximal twist,
which has been demonstrated in general if m_q >> a \Lambda_QCD^2, holds even if
m_q ~ a \Lambda_QCD^2, as long as one uses an appropriate non-perturbative
definition of the twist angle. We demonstrate this with explicit calculations,
for arbitrary twist angle, of all pionic quantities that involve no more than a
single pion in the initial and final states: masses, decay constants, form
factors and condensates, as well as the differences between alternate
definitions of twist angle. We also calculate the axial and pseudoscalar form
factors of the pion, quantities which violate flavor and parity, and which
vanish in the continuum limit. These are of interest because they are not
automatically O(a) improved at maximal twist. They allow a determination of the
unknown low energy constants introduced by discretization errors, and provide
tests of the accuracy of ChPT at NLO. We extend our results into the regime
where m_q ~ a^2 \Lambda_QCD^3, and argue in favor of a recent proposal that
automatic O(a) improvement at maximal twist remains valid in this regime.
