Divide and Conquer
You can make advanced segments with the grouping tools available in the segment builder. It's important to note the difference between "And" and "Or" groupings to make the most effective segments.
"And" means "all these conditions must be met". An example would be, "visited site in the last 30 days" AND "made a purchase". This would return all users who visited site in the last 30 days and also made a purchase. It would NOT return users who only visited the site in the last 30 days. It would also not return users who only made a purchase.
"Or" means "any of these conditions must be met". From the above example, "visited site in the last 30 days" OR "made a purchase", would return users who made a site in the last 30 days. It would ALSO return users who made a purchase.
So be careful when using "Or" statements. Sending an email to a segment that has: "Is subscribed (to email) is true" OR "users who made a purchase in the last 30 days", would send to ALL your subscribed users, based on your first "or" condition. You most likely would use an "and" here instead.
Watch out for unnecessary groupings! Looking at the example below:
You may have been trying to find users who had number of opens greater than or equal to 0, for both of those emails. However, the extra grouping actually makes this: users who opened the first email and not the second one, or vice versa. You should instead try one grouping, with the "greater than or equal to" condition.