Masculine and feminine are noun categories, not linked to physical gender (sex). Any noun can be one or the other and nothing about the nature of the noun determines that. It is something one must simply memorize. There may be patterns that help indicate the grammatical gender but that is not absolute.
Becky, just remember that in Irish, "an" and "na" mean "the" and that there is no word for "a" or "an". "A book" is just "book", "an apple" is just "apple". It is unfortunate that most of these explanations use grammatical terms that are so easily forgotten after we have left school.
In this case the word is aspirated or in Irish, lenition. There are so many reasons for the word to aspirate, I wouldn't focus all your energy on learning those rules. They will come easily the more you work at it. This site will explain it better than I can. Just keep at it, I mess them up all the time. http://nualeargais.ie/gnag/lenition.htm
The difference between them is mainly in emphasis; your first example is a simple statement of fact, and your second example would typically be said only when pointing at the peach. A typical exchange might be:
- A: Where’s the peach?
- B: The peach is in the fridge.
- (A goes into the kitchen and looks inside the fridge.)
- A: I don’t see it — are you sure that it’s in there?
- (B goes into the kitchen, looks in the fridge, sees the peach, points at it, and says:)
- B: There is the peach in the fridge.
The Irish sentence can be translated into either English sentence; my comment was focused on the difference between the English sentences, which was the question in your comment.
Note that that difference in emphasis would not necessarily apply if the subject were indefinite; both “A peach is in the fridge” and “There is a peach in the fridge” are typically used as simple statements of fact. The “There is …” form with an indefinite subject could be used emphatically (with the emphasis on is rather than on there, without accompanying pointing), but that wouldn’t be its usual use.
Note that if the Irish sentence were intended to have emphasis, it would have a slightly different form — for example, Tá an phéitseogsa sa chuisneoir or Tá an phéitseog féin sa chuisneoir would emphasize the peach (e.g. rather than the pear), Tá an phéitseog sa chuisneoirsean or Tá an phéitseog sa chuisneoir féin would emphasize the fridge (e.g. rather than the fruit bowl), etc. This is why the Irish sentence above has been given a translation without emphasis in English.
I'm doing the "listen and write it in english" version on the placement test and put féteog (cocktail), if you've not done the course it's really not fair to put homonyms on the placement test but only accept one of them as correct, the two words are indistinguishable from each other in an aural question
"the peach" and "a peach" mean very different things, in both Irish and in English.