Does it affect grad school admission decisions if you have a few F's?
I can speak to both sides of this question: a student who got an F on his transcript, and as someone who reads a lot of graduate applications. The answer is: it depends. In my case, the F came in my Freshman year, when I was doing fairly poorly across the board. I was not prepared for college courses and the study time required to do well in them. Id more or less coasted through High School without muck difficulty, so I had not developed good study habits. It took me a couple of quarters of college, and the kick-in-the-pants of getting that F to motivate me, but I figured it out and did better, ending up with a pretty good GPA. I also did the things one needed to do to make myself an attractive candidate for grad school. I retook the class as did well in it, and I got a lot of quality research experience. In the lab science programs I applied to, research experience of significant duration and high quality is always key. The committee(s) that accepted me, were willing to overlook my weak start in college because I rallied and then checked the other boxes that were important to them. From the other end, there is generally not any one thing that will be disqualifying. Admissions Committees look at the whole candidate record and try to figure out whether that student will thrive in their graduate program. There are likely to be some weaknesses in most applications, but overall, who is the strongest applicant? And if, as was the case for me, there is a good explanation for the low grades - it can even been seen as a sign of growth, provided that you did grow and do better after that.