Assertive communication means stating your needs, opinions or boundaries clearly and directly while still genuinely respecting the other person, and it is meaningfully different from aggression, which pushes your own view or need forward at the expense of the other person’s feelings or perspective, even though the two can look similar on the surface if you are not paying attention to how they are actually delivered.
Use clear, calm, direct language centred on your own experience, I need, I think, I would prefer, rather than accusatory or demanding language pointed directly at the other person, such as you always or you need to, since the first frames your position as your own honest perspective, while the second tends to trigger defensiveness almost immediately regardless of how reasonable the underlying request actually is. Maintain a calm tone and open, non-confrontational body language throughout, since the exact same words genuinely land very differently depending entirely on how they are delivered, a firm sentence said calmly reads as confident, while the same sentence said sharply reads as aggressive.
It is completely fine, and often genuinely necessary, to be direct and firm about what you need, assertiveness is not about being soft or apologetic, it is specifically about being clear without being harsh toward the other person in the process. Practise this in lower-stakes situations first, a small preference at a restaurant, a minor scheduling need, since assertiveness is a skill that becomes noticeably more natural and comfortable the more consistently you actually use it in everyday, low-pressure moments.
Example: A student who needed to push back on an unfair task distribution in a group project used calm, direct I language, stating specifically what they needed rather than what the group was doing wrong, and found the group responded far more openly than when they had previously stayed silent out of fear of sounding aggressive.
One practical tip: Use clear I language centred on your own need, rather than accusatory you language directed at the other person, and pair it with a calm tone, since the same firm request lands as confident when delivered calmly, and as aggressive when it is not.