1. d. Aspiration
Voiced stops (in English) are never aspirated.
Voiceless stops are sometimes aspirated and sometimes not.
These voiceless stops will be aspirated:
a. Word-initial, regardless of stress:
tap, cat, Topeka (stop precedes an unstressed vowel), command (ditto)
[thQp] [khQt] [th«pik«] [kh«mQnd]
b. Intervocalic (between 2 vowels) but only when preceding a stressed vowel.
meticulous, repair, recalcitrant, return

2. See Table 5-2 (p. 96) of MacKay for a nice summary with examples.
These voiceless stops will be unaspirated:
a. Following /s/
stop, skate, stick, stare, spike
b. Intervocalic, preceding an unstressed vowel
napping, camper, sicken, supper, thirsty
(Note: Sometimes these are unaspirated, sometimes they are lightly aspirated.)
See Table 5-2 (p. 96) of MacKay for a nice summary with examples.

3. Voice Onset Time (VOT) [phɑ] [bɑ]
VOT ~0 ms
VOT ~85 ms
voicing onset
voicing onset and release ~ simultaneous
release
VOT = Interval between articulatory release and onset of voicing.

4. Voice Onset Time (VOT) [phɑt] [spɑt] VOT ~10 ms VOT ~85 ms
voicing onset
release
Very short delay between release and voicing onset (~10 ms)

5. [spɑt] (unaspirated [p])
With [s] edited out

6. /p/ precedes stressed vowel (aspirated)
pack [phæk] capping [khæpɪŋ]
(aspirated [p]) (lightly aspirated [p])
/p/ precedes stressed vowel (aspirated)
/p/ precedes unstressed vowel (unaspirated or lightly aspirated)