54 Science Research Writing Th is is true even if the thing or person has not been mentioned before, for example, in the following sentences: I arrived at Heathrow Airport but the check-in was closed. I bought a new computer but the keyboard was faulty. check-in and keyboard need the because as soon as Heathrow Airport is mentioned, the speaker and listener know about and therefore share check- in; as soon as a computer is mentioned, they share keyboard. Similarly, in the sentence: He lit a match but the fl ame went out. mentioning a match automatically creates the concept of fl ame in the reader's mind — and this shared understanding is marked by the use of the. Similarly, if we were in the same room and I told you to look up at the ceiling, you wouldn't ask me 'Which ceiling are you talking about?' because it would be obvious; we would share it. Did she get the job? (the job we both know she wanted) I'll meet you in the library later. (the library we normally use) Here are some more useful rules: USE THE IF THERE IS ONLY ONE POSSIBLE REFERENT We removed the soft est layer of membrane. Cairo is the capital of Egypt. Th e opening was located in the centre of each mesh. Government policy is committed to protecting the environment. Th e sun's altitude is used to determine latitude. USE A IF IT DOESN'T MATTER or YOU DON'T KNOW or YOUR READER DOESN'T KNOW WHICH THING/ PERSON YOU ARE REFERRING TO.
Methodology — Grammar and Writing Skills 55 A 35 ml brown glass bottle was used to store the liquid. (It doesn't matter which 35 ml brown glass bottle was used.) Th e subject then spoke to an interviewer. (It doesn't matter which interviewer/I know which one but you don't.) It works on the same principle as a combustion engine. (It doesn't matter which combustion engine.) Sometimes the choice of a or the changes the meaning of the sentence completely: (a) Th is eff ect may hide a connection between the two. (Th ere may possibly be a connection between the two but if there is, we cannot see it.) (b) Th is eff ect may hide the connection between the two. (Th ere is defi nitely a connection between the two but we may not be able to see it because of this eff ect.) Here's another pair in which the choice of a or the has a signifi cant eff ect on the meaning (∅ is used here to indicate the plural of a): (a) Th e nodes should be attached to ∅ two adjacent receptor sites. (Th ere are many receptor sites and any two adjacent ones will do.) (b) Th e nodes should be attached to the two adjacent receptor sites. (Th ere are only two receptor sites.) Th e best way to use the information you have just learned is to take a paragraph from a research article that you are reading and use the information in this grammar section to work out why the writer has chosen each instance of the or a, or why the writer has not used any determiner before a particular noun. Another important point to note about the use of a, the and ∅ is that they can all be used generically, i.e. when expressing a general truth: Th e electroencephalograph is a machine for measuring brain waves. An electroencephalograph is a machine for measuring brain waves. Electroencephalographs are machines for measuring brain waves. One last note: a is used before consonant sounds, while an is used before vowel sounds. Sound, not spelling, is important here, so we write an MRI scan because the letter 'M' is pronounced 'em', but a UV light because the letter 'U' is pronounced 'yoo'.