How do i clear up more space on my htc hero and can my apps go on my sd card

You can clear off whatsapp database , also check out if you have multiple whatsapp videoes , as videoes occupy more space. If you have an sd card, dont forget to move apps like facebook to the sd card. For moving apps to sd card, go to settings, then apps, select a particular app, and mostly at down you will get an option of move to sd card.
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this issue appears to have nothing to do with how much space you actually have, whether it's on the SD Card or on the phone - it appears to be a big HTC Mail bug that's just crushing my phone as well, no matter what I purge or reset - there is a long thread here about it -forum.androidcentral.com/.../12215-htc-mail-memory-leak-bug-phone-storage-getting-low-2.html

it's very frustrating and will probably cause me to take back the phone

yup you can replace a larger sd memory to your phone.just buy it on the phone shop or you can erase unnecessary files on it or you can just disc clean up on a pc

searcht eh android market for apps 2 sd or similar, then install and it will move all apps that can be moved to the sd card, some of the file statys ont eh phone however.
also completely clear your internet history and cache.

download 3 apps. cache cleaner cleans temp mem, task killer shuts down unused open apps, memory booster keeps ur phone from stuttering, and apps to sd moves apps to your sd card to free memory.

Basically either clear all your messages and then try. If it still says the same message then you will have to clear applications or mulitmedia from your phone to free some more space. I suggest you delete some applications as normally multimedia is stored on your sd card.

Easiest way is to download an app like astro file. (free) it allows you to look at the sd card and delete like a regular computer folder would

the htc manuel is contained in the sd card, download the linda file manger app and use it to open the pdf file on the sd card

is the application that you downloaded is compatible with your unit? If so just insert the memory card and open it thru your unit