The preposition is through.The noun 'tunnel' is the object of the preposition.Through
"By" can be an adverb, as in "We watched the train go by." "By" can also be a preposition, as in "We stayed in a cottage by the sea."
Yes, it can be. But it can also be an adverb ("Come aboard"). It refers to riding in or on a vehicle, such as a ship, plane, train, or bus.
If you mean 'What preposition would you use to describe train travel?' the answer is 'by': 'I will go by train when I visit my aunt in Edinburgh.' However, 'train' has other meanings, for example, to prepare for something. In that case it is a verb, not a noun. So you would say 'The athlete will train for the Olympics.' You will get a much better answer if you express your question as a complete sentence. It is also more considerate towards the people who you hope will help you. I have made two guesses at what you might mean, but both of them may be wrong. I should not really have to spend time guessing what you mean.
Yes, it is one word "aboard" -- as an adverb or preposition, it means "on board" a vehicle such as a bus, train, plane, or ship.
No, then is not a preposition. It is a conjuction.Than is a preposition.
No, then is not a preposition. It is a conjuction.Than is a preposition.
If is not a preposition. It is a conjunction.
No, a preposition does not always have to be followed by a prepositional phrase. In some cases, a preposition can also be used on its own to show a relationship between two elements in a sentence.
A preposition typically introduces a phrase that provides additional information in a sentence. It is followed by a noun, pronoun, or gerund. For example, in the phrase "in the house," "in" is the preposition and "the house" is the object of the preposition.
At is a preposition. Anything that can be ___ the box is a preposition. For Example: At the box.
No, "together" is not a preposition. It is an adverb that is used to indicate two or more people or things being in one place or gathered as a group.