1.
The problem here is that ph < 7.0 is always true whenever ph < 4.0, so the
program will never get to this if statement.  There are many ways of fixing this
but two possibilities are:

if ph < 3.0:
    print "%s is VERY acidic! Be careful." % (ph)
elif ph < 7.0:
    print "%s is acidic." % (ph)

Another, slightly more verbose option (and not as nice) is:

if 3.0 <= ph < 7.0:
    print "%s is acidic." % (ph)
elsif ph < 3.0:
    print "%s is VERY acidic! Be careful." % (ph)

2. 
def firstprimes(num):
    """Prints out all the primes less than num"""
    current = 1  # the current number we're checking
        
    while current <= num:
        if isprime(current):
            print current
        
        current += 1

(Could also have done with a for loop)