I have a couple of questions to ask.
1) What should one do if he does a mistake in congregational prayers like a mistake requiring Sujood Sahv because I can't do that as I am praying after an Imaam? Sometimes I don't completely erect after Rukoo' or don't take a proper break between the 2 Sujood as sometimes I am but late to react to his Takbeer.
2) Satanic type of thinking always come in my mind tell me what should I do? Questions that I don't have answer to come in my mind and how do I get rid of them? Please I don't want to lose my faith. Help me and pray for me.
3) I have Al-hamdulillaah, explained Islam to a boy in such an influential way that he is about to embrace it. Now he says its a law in his country (Finland) that religion can't be changed before 18 I tell him to do it he says he might but after 18. Secondly he still has doubts so can you please give me a list of sites that have clear proofs of the truth because that will help me and him as well. Thirdly is there any reward for me if he converts?
4) I love Allaah and Islam and everything in my religion but why do wrong thoughts come in my mind to show that my religion is wrong and others are better although I know they are not. And a thought that scientists are proving evolution is sickening me because I think if man was evolved Nauzobillah Quran is wrong? Well I know they are nowhere near proving it but can you help me any further.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If a person who is led in prayer forgets anything in his prayer, he does not have to prostrate for forgetfulness because the Imaam acts on behalf of the people whom he leads in prayer, with the exception of one who neglects any of the obligatory elements of the prayer. Ibn Qudaamah said: 'If a person who is led in prayer forgets, then according to the correct opinion of the scholars he does not have to prostrate for forgetfulness.'

As regards the Tuma'neenah in prayer (which is defined below), if the person being led in prayer does not observe it whilst in Rukoo' (i.e. bowing), or when standing up from the bowing position, or prostrating, or sitting [between the two prostrations], then the Imaam does not bear any responsibility on his behalf regarding the above, because Tuma'neenah in prayer is one of the obligatory elements of the prayer and the Imaam is not a substitute for the people he leads in prayer regarding the mistakes they make in the obligatory elements of the prayer.

An-Nafraawi from the Maaliki school, said: 'The Imaam does not bear any responsibility on behalf of the people he leads in prayer concerning the Raka'h, or prostration, or bowing down, or Tuma'neenah in prayer, or saying the Takbeerat-ul-Ihraam (saying Allaahu Akbar to start the prayer). The only thing he can bear on behalf of the people he leads in prayer are the things that only require prostration for forgetfulness.'

Therefore, if a praying person did not observe Tuma'neenah in prayer, like when bowing down, or prostrating, or standing up from the bowing position, or sitting between the two prostrations, then he has to follow the Imaam and when the Imaam concludes the prayer, then he stands up to make up for the Raka'h in which he did not observe Tuma'neenah. 

As-Suwayfi from the Shaafi'ee school said: 'The praying person is obliged to make up for the act of the prayer in which he did not observe Tuma'neenah when he is an Imaam or when he is praying individually, but if he is led in prayer, then he has to follow the Imaam and when the Imaam concludes the prayer, he stands up to make up one Raka'h.'

A question has to be asked, what is the extent of Tuma'neenah in prayer? The school of Imaam Ahmad consider that there are two ways of observing it:

1) Tuma'neenah is to observe tranquility [while performing an element of the prayer] even for a short time. So if one observes tranquility he has really observed Tuma'neenah.

2)   Tuma'neenah for a duration in which one could say the obligatory legislated mention of Allaah, so if one did not observe Tuma'neenah for this long, then he has not observed Tuma'neenah. The obligatory mention in Rukoo' and Sujood (prostration) is to praise Allaah once 'Subhaana Rabbia Al-A'laa or Al-'Atheem' (depending on the position), or to say 'Rabbighfir lee' (O Allaah! I seek your forgiveness) once when sitting between the two prostrations.

As regards the boy who wants to embrace Islam, you have to know that you will get the reward for calling him to Allaah. It is confirmed that the Prophet said: "I swear by Allaah! If Allaah guides one single man (to Islam) by your efforts, that will be better for you than possessing an abundant number of red she camels (a very expensive breed of camels)." [Al-Bukhari and Muslim] The Prophet further said: "Whoever calls for any act that leads to the right path gets the reward of all the people who followed him.  However, this does not reduce anything from their (the people’s) reward." [Muslim] So you have to encourage him to embrace Islam immediately and not wait until he becomes eighteen; it is not permissible to wait until that time; rather he has to take the initiative to embrace Islam now. It is not a condition for the validity of his becoming a Muslim that he announces his conversion. So he has to become a Muslim and perform the religious obligations even in secret and when he reaches the legal age, then he can change his previous religion with the religion of Islam in official documents, if he so wishes, and proclaim his conversion.

For your other questions, please read Fataawa: 86235 and 83754.

Allaah knows best.