Category 1
fr_gh_t_n
frighten
ci_il_ze
civilize
_iv_rs_fy
diversify
s_ee_en
sweeten
v_sua_iz_
visualize
_i_p_ify
simplify
_ar_on_z_
harmonize
s_m_ar_ze
summarize
cap_t_li_e
capitalize
st_ai_g_te_
straighten
s_mb_l_z_
symbolize
_al_i_y
falsify
l_n_th__
lengthen
en_rg__e
_or_i__en
forbidden
a__lo__z_
apologize
cl__si_y
classify
_n_lyz_
analyze
__mp_n
dampen
_do_iz_
idolize
b__ut_f_
beautify
__ri_y
purify
m__or__e
memorize
mistaken
BONUS:
___w__
(hint: something you find outside & has 6 letters)
flower