The 'd' is indeed optional. ^^
You won't hear a difference in spoken Dutch, but the distinction does matter in written Dutch.
In informal texts, 'hou' is preferred over 'houd' and in formal texts, it's the other way round.
You can also let go of the final 'd' in the third person singular and in questions with the first, second and third person, with these verbs:
- "glijden" (= to glide), "uitglijden" (= to slip)
- "rijden" (= to drive)
- "uitscheiden" (= to cut it out)
- "snijden"(= to cut).
- "Rij(d) jij?" = "Are you driving?"
- "Glij(d) niet uit!" = "Do not slip!"
The comment on the verb preceding the subject only applies to je/jij.
- Ik zwem - Zwem ik?
- Je/Jij zwemt - Zwem je/jij?
In other words when the verb precedes je/jij the same conjugation is used as what you use for ik.
Yes, since Dutch does not have a continuous similar to English. Ik zwem can translate to both the simple present and the present continuous.
Ik ben aan het zwemmen indicates that it is happening at this moment and hence can only be translated using the present continuous.