Well, interpersonal relationships are always a good place to start for subplots, especially in first person. The limited perspective make those the easiest to pull off. Whatever the subplot you want to add, you should be able to implement it regardless of the perspective. The only thing is that you have to limit how they're developed through the vision and intuition of your MC. Unless you have an omniscient main character... which would probly open a whole other can of crazy!
I agree. Perspective shouldn't limit you. You don't have to tell the entire story through the eyes of one character if they are not directly involved in the subplot.
if you stick completely to that one first person narrator, keep in mind that you won't be able to let the readers in on whatever is happening beyond that character's ability to see, hear, or know about it... so subplots will only be able to be related to that one person's experience...
I think, but I'm not sure, subplots when dealing with the first person usually deal with the mc's friends or family. That way she/ he can be easily updated on what's going on with the subplot. I read this interesting old ya book recently - The Creep by Susan Dodson which was about an underdeveloped 16 year old girl allowing herself to be used as bait by the police in order to catch a child molester. The subplot involved her mother's budding romance with the police detective that was training he girl, and the mother coming to terms with being molested herself as a child. I'd look to the mc's friends, family or boyfriend/girlfriend for the subplot. Anyone that has access to the mc on a semi-daily basis.
Subplots comprise the conflict and the tension of a story, and are therefore not defined by the POV. Please read What is Plot Creation and Development?
I think that personal relationships (girlfriend, parents, siblings etc) are a great subplots. You can tell the reader through the thoughts of the main character how he/she feels about the events happening in the relationship to give more depth to the subplot. You could also give the main character certain goals, that are not directly related to main plot, and have him/her accomplish them through the story; giving him/her thoughts the same way as above.
As Mom said, first person will limit you to subplots involving the single character. Third person allows for more, but if you're working first, it'll need to be something effecting that person, or a secondary goal he/she needs to accomplish.
Sticking to a single narrator doesn't inherently limit which subplots you have access to. However, it does limit how easily you can access them. For example, you may have a subplot about murders taking place in the MC's town. It would be easy to introduce this subplot by leaping to the POV of a journalist or detective at the crime scene. But, you could get the same story across by sticking with the MC and having him/her see it on the news, read about it in the paper, or ride past the crime scene on his/her way to work or school. A story with multiple narrators can leap into subplots and then leap back to the main plot at will, while a story that sticks to one narrator has to be a little more tactical. The latter is usually subtler, but it doesn't have to be. For example, you could introduce the murder subplot by having the MC get attacked by the murderer... but that runs the risk of treading into "main plot" territory, as it is a little too "close" to be considered a peripheral event.
