Genesis 42:7

And Joseph saw his brethren, and recognised them; but he made himself as a stranger in their eyes, and spake hard words to them, and said to them, Whence come yon? And they said, From the land of Kenaan, to buy corn.

He recognized … Yoseif recognized. He knew immediately that they were his brothers, but only after he examined them more carefully did he know who was who.

AND JOSEPH SAW HIS BRETHREN, AND HE RECOGNIZED THEM. Immediately as he saw them he recognized them, and he feared lest they recognize him. And he made himself strange (‘vayithnakeir’) unto them by putting a mitre upon his forehead and part of the face, thus disguising himself, just as it is said concerning the wife of Jeroboam, Arise, I pray thee, and disguise thyself, that thou be not known to be the wife of Jeroboam, in a physical sense. Joseph disguised himself by placing his mitre over his face. Further on, Ramban mentions a second interpretation. See Note 126. and it further says, For it will be, when she cometh in, that she ‘mithnakeirah’ (will pretend to be another woman)., Verse 6. It may be that the word vayithnakeir here means that he made himself strange by his words, speaking to them harshly and asking them in anger — as if it were not customary to come before him to purchase food — “From where do you come to appear before me?” And they said, From the land of Canaan to buy food. When they mentioned this to him, it then became clear to him that they were indeed his brothers. This is the meaning of the expression, And Joseph recognized his brethren, which is mentioned a second time to indicate an additional sense of recognition and knowledge of the truth with respect to them.
Now Rashi wrote in explanation of the word vayithnakeir: “He made himself like a nochri (stranger) in conversation by speaking harshly to them.” According to Rashi’s opinion, the word vayithnakeir signifies that he spoke to them as a man who is a nochri (stranger). But this is not correct. having the same root as heker (recognition), except that the word here changes its meaning so as to indicate its opposite. See Rashi on Exodus 27:3, where he says that there are many such words in the Hebrew language. In the case before us, the word vayithnakeir would thus mean that Joseph made himself unrecognizable by speaking harshly to them. See my Hebrew commentary, p. 232.

מאין באתם?; he pretended not to know who they were.

ויכירם, he recognised them collectively as being his brothers, without being able to recognise who each brother was.

ויתנכר אליהם, he adopted a manner totally uncharacteristic of him, by speaking arrogantly, masking his true voice, etc, apart from the fact that he spoke to them in Egyptian having everything he said translated into Hebrew by an interpreter.

קשות, so that they would not recognise his voice.

וירא יוסף...ויכירם, our sages explain that the fact that Joseph had left home when he did not have a beard yet, whereas his brothers already had beards accounts for the fact that whereas they did not recognise him, he recognised them. (quoted by Rashi) It is possible that they would have recognised him in spite of his having grown a beard if they had not seen the man confronting them dressed in the uniform of the highest ranking nobles of the land. They could not imagine that this man could be Joseph even if there had been points of resemblance. The idea that someone who had been sold into slavery had risen to such power was too mind-boggling for them.

ויתנכר, Joseph moreover deliberately misrepresented himself to prevent their suspecting that it might be him.