Hi Akiva,

You did not really specify what you want to understand, so I will try to explain Rebbi Yishmael overall.

The Posuk says that if someone's animal goes ahead and eats someone else's crop, in his friends 'Sadeh', he will pay from the best of his 'Sadeh'. The simple way to understand this, is what is called a Limud of Gezera Shava. When we are trying to understand a word or phrase which is not clear, we can learn from a different place that is clear cut, that the same meaning applies to the same word in the place which is less clear. The Gemara uses the phrase Gezera Shava too.

The Shita Mekubetzes (in order to answer a different question), brings an opinion that in cases like this, where the two similar words are in the same Posuk or in the same paragraph or issue, this is not a Gezera Shva, but rather a different kind of Drasha, or a different kind of way to understand the unclear phrase, called 'Yilmad Sasum Min ha'Meforash'. We can learn the unclear from the clear. This is not from the phrase or similar word, but rather from the context of the word.

In any case, the Gemara makes thing a bit difficult, since it seems that the Posuk in the 'unclear' part, is clearly talking about the Mazik.

To this Rebbi Yishmael answers, that the Limud or Drasha is a combined one. There are cases where the Mazik will pay the estimated Nezek according to the best of the Mazik, as learned from the Gezera Shva, while in others he will pay in accordance to his 'best' like the Pshat of the Posuk is.

Normally we use the Idis of the Nizak to estimate from which property the Mazik will pay. In a case the Mazik does not have a property equal to the best property of the Nizak, only one that is better and another that is worse, in this case he will have to pay from the better property even though it is more than the usual Chiyuv, rather than paying from the worse property.

Let me know if I missed anything!

Kol Tuv,

Aharon Steiner