People who are practicing aikido are not competitors - they are helping each other in everyday practicing. The one who is doing the technique is tori and the one who is attacking is uke. Tori is doing technique four times than they change - the one who was uke is now tori and the one who was tori is now uke. In that way partners are helping each other to practice and progress.
Tori should always adapt his strength and speed to his partners capabilities. The same apply for the uke - he should never use all his strength to attack the beginner - he should adapt his attack to the skills of his partner.
Uke should never resist or try to block his partner's technique but to halp him to practice, learn and improve his technique. Partners should turn attention to each other on possible omissions in the techniques. Blocking the technique is a certain way for injury.
Usually one change several partners during the training, so he is able to practice with people who are stronger or weaker, bigger or smaller, and more or less skilled than himself. Learning to adapt to the partner is an important part of aikido training because aikido training is not focused only on technique but also to build character and personality.