Albert Barnes Commentary Isaiah 8:11

"For Jehovah spake thus to me with a strong hand, and instructed me not to walk in the way of this people, saying," — Isaiah 8:11 (ASV)

For the Lord spoke thus – He spoke what immediately follows in the next verse. He warned him not to unite in the alliance with foreign kingdoms that the nation was about to form.

With a strong hand – Margin, ‘With strength of hand.’ That is, when the hand of God urged me. A strong prophetic impulse is often represented as being produced by God’s laying His hand on the prophet, or by his being in this way, as it were, urged or impelled to it; (Ezekiel 3:14): The hand of Jehovah was strong upon me; (2 Kings 3:15): And it came to pass, that when the minstrel played, the hand of the Lord came upon him; (Jeremiah 20:7): O Lord, you are stronger than I, and have prevailed; see also Ecclesiastes 2:24; 1 Kings 18:46; 2 Kings 3:15; Ezekiel 33:22; Ezekiel 40:1; compare the Introduction, section 7.11.(3).

The meaning is that the prophet was strongly, and almost irresistibly, urged by the divine influence to say what he was about to say.

That I should not walk ... – That I should not approve, and fall in with, the plan of Ahaz and of the nation in calling for the aid of the Assyrian armies.