The correct ways to pronounce them are 那个 nà ge 是 shì People from north part of China usually pronounce 那个 as "nei ge" and people from southern China usually say "si" instead of "shi". It's just the different accents

那个: the orthodox pronunciation is na ge But a huge number of the native Chinese speakers would say ' nei ge ' which is the phonaesthetic pronunciation of 那一个 （na yi ge ）. Anyway 那个 and 那一个 are interchangeable in most （but not all ）situations. My advice is for everyday use, nei ge is preferred but for more technical use, there should be a lexical difference. 是: the standard pronunciation is shi.

是 is just pronounced "shi" some southern chinese people pronounce it "si" because of accent... 那个 is pronounced "na ge" BUT if you repeat it very fast , you will normally arrive "nei ge"...... 99% chinese pronounce 那个 "nei ge" its not an accent its a kind of "lazy way" in every language, we can find some phenomenon like this: "what are you doing?" the "t" of "what" normally we dont pronounce it if we say this fast. in french, i know two phrases like this: "tu es ou?"(where are you?) people say "t'es ou"? "je ne sais pas"(I dont know) people say "ch sais pas" "je suis d'accord"(I agree) people say "chsuis d'accord" 那个----na ge----- nei ge exactly the same reason

"那" goes for "nà"; "是" goes for "shì"

正确的应该是： 那个--> nà·ge 哪个--> nǎ ge 是 --> shì