Your biggest difficulty in Japanese grammar…

Fellow students of Japanese:

Which of the following four problem areas of Japanese grammar do you find, or did you find, to be the most troublesome:

1. Correct usage of fundamental particles such as が, は, を, に, も
2. Correct usage of if/when structures such as 行ったら, 行けば, 行くなら, 行くと, 行く時
3. Correct usage of auxiliary verbs くれる, もらう, あげる, いただく, やる
4. Correct usage of nominalising words こと and の (の is included here only as a nominaliser, not as the possessive の)

To rephrase, if you could have had additional help with any one of these areas, which one would it have been?

EDIT: 5) Causative, passive, and causative-passive is now included as option 5 by popular request.