Hi Peg, after the preposition 'di' (and others prepositions) it is often used the infinitive, as a grammar rule. In this case it is used the infinitive to express the meaning of 'to see' in a general sense, in an indetermined way...: "everything you see is in stripes". The infinitive is often used to express something indetermined. The sentence you have written in Italian: "È sicuro di vedere a righe?", would be translated in English: "Are you sure that you see in stripes?" The sentence you have written in English: "It is certain that you see in stripes", would be translated in Italian: "È sicuro che vedi a strisce". As you can see, after the conjunction "che" it is not used the infinitive. I hope this is of some help ^_^