This is the 877th most frequent Russian word.
внимательно
Attentively.
adverb, describing how the action of reading was performed
Они внимательно читали инструкцию перед началом эксперимента.
They carefully read the instructions before starting the experiment.
adverb, modifying the verb 'listen' to show the manner of listening
Слушайте внимательно, это важно.
Listen carefully, this is important.
adverb, indicating the manner of observation