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i need to write more descriptively. can someone please help me?

Some things that always helps me make better and more descriptive posts are:

1. Make questions for your character to answer as you make the post. Some possible questions might be (all asked in character): What am I thinking? How do I feel? What am I seeing? Hearing? Tasting?

2. Describe what you do know is happening. An example might be if you are on a beach, you could feel the light ocean breeze coming of the water. The bright and pleasant (or harsh, depending upon your character), sunlight as it warms you up.
Unless it is your role play or your setting, try not to change the area. This means that if you don't know what the setting is around you, and your not the one making the role play story, don't mention the sun or weather. If you need to, PM the creator of the role play (or host of the story) about what the setting is for you to make a proper post. You can even do this in out of character post if you wish.
Also, if the setting is a sunny day on the beach, don't suddenly add in gray storm clouds on the horizon, unless you are in charge of the storyboard. The reason for this is that you would be controlling more than just your character. You would be pulling a little god-mod there by controlling the surroundings. Just something to look out for.

3. If you are doing something, you can make a post a little longer by adding in descriptions on how you do it. Maybe you shrug your shoulder back as you swing your tennis racket at the ball, slamming it even harder to the other side. The tennis ball could spin along the grass as it slips by you, whizzing by your head by mear inches.
This part is solely up to you on how far you wish to take it. You can keep it simple, which may work for most role plays, or you can go all out and add in as much detail as you can.

By using all three of these methods, you could probably turn a one sentence post into at least a paragraph or two. For the record, unless you are having a conversation, dialogue only, then I do not recomend doing just a one sentence post. It is very short and makes it harder on your role playing friends to make a good reaction to your post. Always try to leave your post open ended. This gives others a reason to respond. If you leave an opening for others, then the role play can go on longer, and will usually be more enjoyable for all involved.