If I am praying and mistakenly I forgot one ayat in the middle of a surah and complete the remaining part of surah (e.g. in surah al-Naas I accidentally skipped ayat no 3 and recited the rest of the surah as normal). So how will be salat main while I am Imam?
Is the prayer valid?
What our brother @Samir Elwan wrote is true but, just to make it clear: scholars made a difference between the verses or surah after reciting the fatiha and the fatiha itself.
Now if you left one Verse from any other surah scholars say that the prayer is valid. Because reciting after the fatiha is sunna and not obligatory. But the fatiha is fard (obligatory) because of the known ahadith!
What to do if you were Imam and this happend?
Usualy in a prayer in congregetion if the imam stops because he fogot a verse or did a mistake if somebody could help him he should recit the correct verse (in Arabic it's called al-fatah 'ala al-Imam الفتح على الإمام) as mentioned in the following Hadith:
Narrated Al-Miswar ibn Yazid al-Maliki: The Messenger of Allah (ﷺ) recited - Yahya (sub narrator) said: Sometimes al-Miswar said: I prayed along with the Messenger of Allah (ﷺ) and witnessed that he recited - the Qur'an during the prayer and omitted something (i.e. some verses inadvertently) which he did not recite.
A man said to him: Messenger of Allah, you omitted such-and-such verse. The Messenger of Allah (ﷺ) said: Why did you not remind me of it?
The narrator Sulayman said in his version: He (the man) said: I thought that it (the verse) was repealed.
[Sunan Abi Dawod]
Now if the imam has nobody to help him out he has two choices either to move to ruku' or to jump the words/verse he forgot if he recalls the rest of it according to this fatwa.
See also about mistaking in recitation during prayer.
And Allah knows best!
The Fatwa said that if you forgot an ayah in surah during salat unintentionally that's doesn't invalidate the salat. And Allaho aa'lm