In Revelation 10:4 John tells us: “When the Seven Thunders had uttered their voices, I was about to write.” The fact that he was ready to write down what he had heard indicates the “thunder” must have been intelligible. John seems to have understood what was said, so what was heard must have been in the sense of John 12:28-29. In other words, those who have ears to hear would be able to understand (Matthew 13:9). However, we are then told that John heard a voice from heaven (cf. Revelation 1:10-13; 4:1; 18:4; 21:5, 15), commanding him not to write what he heard. This seems to contradict what John was told earlier (Revelation 1:11, 19). Therefore, we need to consider closely what John was told to do in place of writing what he heard said.
On the face, telling John not to write what he heard is somewhat surprising. However, Paul does conclude that much of what is written in the Apocalypse describes things that John had no adequate words to communicate properly (cf. 2Corinthians 12:4). Therefore, perhaps, although John was able to discern what was said, he may not have understood completely, and the Lord knew this was so. Often we will find that truth cannot be communicated through simple logic. Apparently, certain trials must be experienced before enlightenment can occur (cf. John 2:22; 12:16; 16:12). Simply because knowledge is spoken doesn’t mean anyone or everyone is able to understand what is said (cf. Daniel 8:26; 12:4-9; compare this with 1Peter 1:10-12).
The Greek word translated not in “write them not” is me (G3361) is a conditional negative. It is not as ou (G3756) which is an absolute negative. I believe the sense of Revelation 10:4 is that John is not to absolutely (ou – G3756) hide what was said, but to hide it on condition (me – G3361) that he is to prophesy what he heard out of heaven (Revelation 10:8-11). Therefore, ultimately the Apocalypse remains an unsealed book (Revelation 5:2, 5; 22:10).
Next, the Angel who stood upon the sea and the earth, while facing Jerusalem, lifted his (right) hand to heaven (Revelation 10:5). The posture that the Angel had taken has always indicated that person was entering into a solemn pact with God, whereby he calls upon God to witness and acknowledge what he says or does and / or promises to honor him, if God will help him in his task (cf. Genesis 14:22). Therefore, the Angel was calling upon God to act with him in what he was about to do to Jerusalem (cf. Genesis 19:24), which was before him, in that he had his right foot on the sea and his left on the earth (Revelation 10:2, 5-6).
The posture of the Angel and his swearing by him who was in heaven recalls another Scripture that speaks similarly:
Daniel 12:7 I heard the man dressed in linen clothes who was above the river. He raised his right hand and left hand to heaven and swore an oath by the one who lives forever. He said, “It will be for a time, times, and half of a time. When the power of the holy people has been completely shattered, then all these things will be finished.”
In Daniel the book is in the hands of Daniel (Daniel 12:4), but here the book is in the Angel’s hand (Revelation 10:2). In Daniel the book is sealed, but in the Apocalypse it is open. In Daniel the Angel is on the waters of the river, Euphrates (Daniel 12:7), but in the Apocalypse the Euphrates is behind the Angel, as he stands facing Jerusalem with is right foot on the sea and his left on the earth (Revelation 10:2). In Daniel the Angel said that it would be for a time, times and half a time, and, when the power of the holy people will be completely shattered, it would be finished. However, in the Apocalypse the Angel said there would be no further delay. In other words the 3 ½ years predicted in Daniel 12:7 must have been complete by the time of Revelation 10:5-6.
 E.W. Bullinger; Companion Bible Appendix-105