I crushed my antibiotics (amoxicillin + potassium Clavulanate). Does it deform in the stomach acid and therefore become useless? Or does the acid have no effect and the amoxicillin goes directly into the intestines and blood stream subsequently?
Because amoxicillin is available as oral tablets, it can be ascertained that it is not significantly degraded to a useless form in the stomach. In fact, it has an oral bioavailability of 95% (meaning 95% of the orally ingested material goes into the blood stream rather than being destroyed in the stomach). The term for an oral medication being destroyed before it enters the blood stream is "first pass metabolism". So oral amoxicillin can be said to undergo minimal first pass metabolism. Oral potassium clavulanate also has a high oral bioavailability.
The accepted answer is correct in some respects, though the assertion that an oral dose can be assumed to be safely crushed is only generally true. The Institute of Safe Medication Practices maintains a list of medications that can't be crushed (available for "free" on some sites, but with overlap in lists like this one). These medications are usually formulated in a way that they will slowly release a large dose over time or are coated to protect the drug from stomach acid ("enteric coated").
Importantly, the amoxicillin/clavulanate you crushed (Augmentin is a common brand name of the medication) is often given in extended release form containing 875 mg amox and 125 mg clav, and should not be crushed *. If that is the case, I would discuss the specific changes to dosing with your doctor or pharmacist that this would entail; generally it's not a big deal, but this site is not the proper forum to advise you.
* From your comment on the accepted response, it seems you surmised this yourself. However, because of some important info left out of the accepted response, I leave this answer as is.