"A fair tide can be carried well past Lands End before the stream sets to the north.“ I know every word of the sentence but can't understand it at all!

"Tide", in this case, refers more to a current or flow rather than the vertical movement. A "fair tide" is a current which is favourable to the ship's direction: one which helps the ship move quickly and easily. A "fair wind", likewise, means a good wind which is not too strong and which is blowing in such a way as to make sailing easy. "Fair" is one of those words you have to be careful of. It can mean several different things, depending on context. Here it means 'good' or 'favourable' (in contrast to 'foul'). When used of a decision or judgement, it means 'equable' or 'impartial'. It can also mean 'only average', 'less than excellent' or 'better than bad': Q: "Did you have a good round of golf?" Compare the following answers: A1: "It was fair." (meaning it wasn't good but not bad either). A2: "It was fairly good" (meaning that it was not the best but probably a little better than average) A3: "It was a fair game." (MIGHT mean the same as A1 but more likely to mean that the result was what was expected and that it was an accurate reflection of the speaker's ability. It could also mean just that nobody cheated!) A4: "Yes, John played fairly, for a change!" (= "John did not cheat, as he usually does!") "Land's End" is the name of the most westerly point of England (it is not just a general term for 'the end of the land'). A ship travelling from the south coast of England northwards needs to avoid some dangerous rocks around Land's End, so a 'fair tide' means a current which avoids these and takes the ship well to the west of them before it turns north. "Can be carried ..." is nautical 'jargon' and means that the ship can keep within the favourable current. In fact, it would be more usual (for a landsman) to think of the current carrying the ship, rather than the other way round! "Before" may refer to time: "It is a little before six o'clock", "Go west for three blocks before turning north.", "the calm before the storm", "Before Christ" (in dates, as in 500BC). but it can also be used for position, meaning "in front of" (usually when looking at the object) or "in the presence of": "He stood before the picture for a long time, marvelling at the detail." "The priest stood before the altar." "Do not put work before your marriage." (= do not make work more important than your conjugal relationship) "I swear before God." (= "I declare [that I am telling the truth] in the presence of God")