tn The word order shows the emphasis: “I am not able, I by myself, to bear all this people.” The infinitive לָשֵׂאת (lase’t) serves as the direct object of the verb. The expression is figurative, for bearing or carrying the people means being responsible for all their needs and cares.
tn The subject of the verb “heavy” is unstated; in the context it probably refers to the people, or the burden of caring for the people. This responsibility was turning out to be a heavier responsibility than Moses anticipated. Alone he was totally inadequate.