How can I deduce long sentences and find their meaning easily

How can I deduce long sentences to find their meaning. I am facing this problem when I am preparing for GRE . As I am not a native English speaker, I feel lost while I am reading those long convoluted sentences, like

It would seem unlikely that evolution should discriminate against sinistral snails if sinistral and dextral snails are exact mirror images, for any disadvantage that a sinistral twist in itself could confer on its possessor is almost inconceivable.

Though I don't have problem with vocabulary, but I feel lost. Is there any technique to understand them easily?

• For what it's worth, as a native speaker this sentence took me a few goes to properly understand. Sep 3, 2017 at 12:20
• "Since sinistral and dextral snails are exact mirror images, evolution is unlikely to have favored one over the other. If evolution has favored one over the other, we do not know why." Sep 3, 2017 at 17:30
• I think the technique is to read slowly and carefully, and make sure you understand the first half of the sentence (before the "for") before progressing to the second half. Sep 3, 2017 at 18:58

Try splitting the sentence into logical pieces. Use the 'little words' (and, for, but, etc) as guides to the relationship of the pieces.

• It would seem unlikely that evolution should discriminate against sinistral snails if sinistral and dextral snails are exact mirror images, for any disadvantage that a sinistral twist in itself could confer on its possessor is almost inconceivable.

Let's start with the word that. Something is unlikely. What is it? Find the piece starting with the word after that that forms a proposition:

• evolution should discriminate against sinistral snails

The structure before the comma now looks like:

• It would seem unlikely that (blah 1) if (blah 2)

The word for after the comma looks like a reason for (blah 1) to "seem unlikely".

Continuing this way and repeating with the smaller units, we end up with:

• It would seem unlikely that (blah 1) if (blah 2), for any disadvantage that (blah 3) could confer on (blah 4) is almost inconceivable.

Try to understand each bracketed piece, then check that they make sense in the larger structure. If they don't make sense, group the words in a different way and try again.