#1
Hello ive lurked around the forums before but decided to finally make an account. Anyways, ive been learning guitar for like 8 months and have gotten some basics down, but as i started learning some of the "generic post hardcore" music like blessthefall and asking alexandria, i noticed they strum a lot of 5 on A and 7 on G. So i figure i have to mute the D string and i do but every time i strum it just sounds floppy. Any advice for strumming, is it just speed? Its just been giving me a hard time haha. Thanks
#2
They're just octaves..

As you know they look like this:

e|---
B|---
G|-7-
D|---
A|-5-
E|---

Both the 5 and the 7 are a D note, except with an octave difference, thus why they're called octaves.
They're quite easy to play actually and just require muting the D string. Typically you mute the D string using the tip of your ring finger. So the finger playing the 7th fret (ring finger) will be muting the D string with the tip of that finger. Your index finger also does some of the muting seeing as it's in a sort of bar position so that should dampen it as it is. Then you also have your middle finger which should sit normally over the string and that will also apply a muting to it. So really you have 3 fingers muting it - index, middle, ring.
Pay most attention to the ring finger however because the majority of the muting comes from that finger.
Keep practicing and eventually you'll wonder why you ever found octaves difficult.
#3
e|---
B|---
G|-7-
D|-x-
A|-5-
E|---

To be more exact, the chord you're playing looks like this. Rake the pick across the D as well, but make sure it's muted with your fretting hand. Just thought I'd clarify that in case that's the problem.
#4
practically using the ring finger is the most convenient. u need to position it till it's like poking the D string from below..