How do I link or copy/paste the Player's Handbook passages to my post?
Pasting isn't always necessary
Mostly you don't need to paste any PHB material into posts. Anyone who can answer already has their own copy to read, so usually you just need to say what you're asking about.
Your first question How complex can my commands be with the conjured animal spell? is plenty clear on what it's asking about, and is an example of a question that doesn't really need to quote or link to the spell's text.
Linking is a nice bonus
If you just want to have a clear link to the information you're talking about, that is a nice bonus, since it saves people time opening their books or finding a link to their favourite SRD. You can do that by linking to any of the online copies of the free portions of the Player's Handbook, which is called the System Reference Document or SRD. There are a bunch of online copies of the 5th edition SRD — choose whichever you like best.
Find the information there that you want to link to, and then copy it's URL from your browser's address bar. Then add it as a link to your question. You can add links by highlighting the words you want to become a link (for example “Conjure Elemental”) and clicking the “chain” icon in the editor's toolbar. Alternatively, you can do it by hand, by typing:
[Conjure Elemental](http://www.5esrd.com/spellcasting/all-spells/c/conjure-elemental/) is a spell that…
Which will look like this in the final question:
Conjure Elemental is a spell that…
Also, compared to quoting the whole text you're asking about, just using a link saves space in the question and makes it easier to read.
If you do want to paste though…
If you really need to point at something in a passage — like you want to say “this line here is the confusing part I'm asking about, see how it's confusing?” — then it can make sense to include a quote of what you're asking about.
You can do that by typing the quote and then highlighting it and pressing the “quote marks” button in the editor's toolbar. Alternatively, you can just do it by typing a
> in front of the quote. Just try to quote the smallest amount of the book you need to be clear — nobody wants to read a whole spell description just to find the one word that you're asking about.
In the editor the quote will look like this:
Who counts as a “companion” for Conjure Elemental? It says (PHB p. 225): > The elemental is friendly to you and your companions for the duration. … but I'm not sure what that means. Do they have to be friendly with me, or just *with* me? For example, would the elemental be friendly to a prisoner we have, or would it attack our prisoner?
which would look like this (but without the outer quote box) in the actual posted question:
Who counts as a “companion” for Conjure Elemental? It says (PHB p. 225):
The elemental is friendly to you and your companions for the duration.
… but I'm not sure what that means. Do they have to be friendly with me, or just with me? For example, would the elemental be friendly to a prisoner we have, or would it attack our prisoner?
Using both is good too
You can link to an SRD reference as well as quote the passage you're asking about. For an example of that, take a look at the question in What counts as a curse?