How do you determine mild sins from serious sins?  What happens if you commit a serious sin before you convert to Islam?

I would like to reiterate that I am not here to offend anyone

I was told today that Muhammed was a sinner yet Muslims claimed that Prophets cannot sin

Where did you 'find it out'? Am asking so as to know how to answer your question. Many people post lies about muslims and Islam. This is just one of them.
About your first question;
whatever a person did wrong before entering Islam is forgiven when he becomes a muslim.

Yeah, once you convert to Islam you are like a new born baby.




Big sins involve murder, shirk, and some few more

fellis:

Where did you 'find it out'? Am asking so as to know how to answer your question. Many people post lies about muslims and Islam. This is just one of them.
About your first question;
whatever a person did wrong before entering Islam is forgiven when he becomes a muslim.

Firstly, I'll try not to post anything without back-ups or lies
Secondly, it's obvious you are a new recruit so you may want to read Chapter 80 in the Quoran before you answer me? Didn't Prophet Mohammad turned his back on a blind man when he was preaching? And didn't he asked for forgiveness? Why did he seek forgiveness?

'New recruit'.
I did not very much like being called that. What type of recruitment are you talking about? Who is recruiting who here? Your choice of words has shown me that your intentions for starting this thread were not good ones.
See, folami, it is not mandatory on you to join those who constantly make trouble on this forum. You can choose to not be a trouble maker; it is not going to take anything away from you. Just a word of advice (Although I think this your username is just one of the multiple ones you have.).
You asked if Muhammed(SAW) sought forgiveness.
He did.
For the sins he committed unintentionally; by mistake or out of forgetfulness. He was a man of high standards and when he asked Allah for forgiveness, it was not at all because he was a habitual sinner, it was for unintented wrongdoing.

About Qur'an Chapter 80.
Muhammad was a very modest man who was full of love for his people and anxious to do good to them and guide them aright and he was strongly self-critical and fearful of bringing the least harm to the weak or the oppressed.
He (Muhammed, SAW) was once involved in a serious conversation with Al walid Ibn Al Mughirah, one of the leading aristocrats of Quraysh, whom he hoped he would convert to Islam.
ibn Umm Maktum, the blind person, stopped by and asked Muhammed to recite some Qur'anic verses for him. Preoccupied with his conversation, Muhammed did not answer. Ibn Maktum insisted untill he interrupted the conversation of the two men, to the severe annoyance of Muhammed. The conversation was abrubtly ended. Muhammed frowned, looked to the blind man and moved on without satisfying his request.
When he came to himself, he began to critisize himself for this treatment of the blind man and soon those verses were revealed to him and he was reproved by them.
He did not turn away because he looked down on, or hated him. He did it because he naturally disliked the interruption.
In later years, he greeted Ibn Umm Maktum, (the blind man) with these words of humility, "Welcome unto him on whose account my Sustainer has rebuked me.".
There.
I hope that btw the time I sent the last post and finished typing this one, that expletives have not been exchanged. . . . . .

@Folami86: Could just turning one's back to a person who was trying to interrupt your conversation be considered a sin? If so what happens to a one who curse a people, or cursed a tree or received the whispering of devil as in three temptations, or the one who did not show mercy to an agonizing theif on a cross, etc?

Be reasonable. Tell us whats noble about these things and tell us whats not noble about mere turning of back? If turning of back deserves seeking forgiveness, then know that the whispering of shaitan must deserve an equal or greater among of forgiveness to be sort!

It is rude for someone to interrupt a conversation. It is also rude to turn your back on someone. And it is also insulting to turn your back on someone, agree?. Isn't insulting a sin? Could that be the reason why Muhammed asked for forgiveness?(he knew he had sin)

In fact, I like felis reply:

fellis:

For the sins he committed unintentionally; by mistake or out of forgetfulness. He was a man of high standards and when he asked Allah for forgiveness, it was not at all because he was a habitual sinner, it was for unintented wrongdoing.

I know my Bible has scripture(s) about insults (James 4: 1-12), so I am sure there must be a Quoran verse about insults

@folami86,
you are deliberately refusing to understand what I am saying. Do not carry on like this, it would not help our conversation(infact I am almost starting to think this whole thing is a joke to you). Why did you make only part of the statement bold? Did you not read the other part; '. . .for sins committed unintentionally; by mistake or out of forgetfulness'.?
Muslims know that as humans, they cannot be perfect in anything or 100% sinless, as only Allah is perfect. They can only be NEAR perfection, so they seek forgiveness for the sins they commit by mistake, due to their imperfect nature, Qur'an 2:186.
At this point, I would like to ask you, folami86, a question. What about the sins you, as a christian commit unintentionally? What does your bible say about them? What will happen to you because of those sins. Do answer this question, as I would like to know about it.
You asked if turning one's back was rude. . .
Na wa for that your question.
Did you read what I wrote properly? How can turning your back in that context be considered rude and insulting? Stop drawing conclusions based on your own biased understanding of what happened. Read the part where I mentioned that he was naturally angry at the interruption again.