Attosecond pulses combined with infrared laser constitute a powerful tool for controlling atomic photoionization and high-order harmonic generation (HHG). We apply the intense-field many-body S-matrix theory to solve such two-pulse excitation problems. The theory can give a clear explanation for the oscillation of ionization probability as a function of time delay between infrared field and attosecond pulses with central frequency lying below ionization threshold at moderate infrared intensities. The HHG assisted by such attosecond pulses is also interpreted. In addition to a known dramatic enhancement of HHG, a harmonic emission from rapid oscillation of bound-state population caused by the counter-rotating wave is presented.