Learn Arabic (Modern Standard): حاولت مرة أخرى.

The word مرة means 'a time' or 'one instance'. The word أخرى means 'other' or 'another'. So the phrase مرة أخرى literally translates to 'another time', which is the standard way to say 'again' in Arabic.

Why is the feminine word أخرى used here instead of the masculine آخر?

In Arabic, adjectives must match the gender of the noun they describe. Because مرة is a feminine noun (indicated by the taa marbuta or ة at the end), the word for 'another' must also be in its feminine form, which is أخرى.

How do we know حاولت means 'I tried'? Couldn't it also mean 'she tried' or 'you tried'?

In written Arabic without short vowels, حاولت can indeed mean 'I tried', 'you tried', or 'she tried'. The context usually tells you which one it is. When speaking, the invisible short vowel on the final ت makes the meaning clear: an 'u' sound makes it 'I tried', while an 'a' sound makes it 'you tried' (masculine).