大 家 好！

Note:

1）“个” is the measure word (classifier) for "月", so four months will be “四个月” in Chinese.

2) "Last year" is 去年， but last month and last week are "上个月" and "上（个）星期".

3) Last year I went to China, we use “了” in past tense.

4) In your context, "this is truth" should be "这是事实".“事实” refers to something that is true. While “真理” refers to the philosophical concept of "truth" , which is normally defined as: a statement about the way the world actually is.

5) “这很难”。In Chinese, degree adverbs such as 很 (hěn) and 真(zhēn）,  can serve as a link between the noun and adjective (you could think of it as a substitute for the verb "to be"), and the sentences could be translated as "(Noun) is (adjective)."

     for example, 我很好 (I am fine），we do not say "我是好”; 你真棒(You are awesome）， not "你是棒".

Hope this can be helpful. 加油！

大 家 好！

RIght

大家好！我叫Oilyn。我已经学习汉语四个月了。我很喜欢中国，我去年来过中国。我妈妈问我：为什么你想去中国？你都不会说汉语。的确是这样，我现在只会说一点儿汉语，因为它很难，我的记性不好，常常忘记汉字怎么写，我的老师建议我要更加勤奋地学习。

大 家 好！

大 家 好！

大家好！我叫Oilyn. 我学习汉语已经有四个月。我很喜欢中国。上去年我在去过中国去了。我妈妈问我: 为什么你想去中国？你都不会说汉语。这是真理这是事实，不过现在我会说一点儿了。可是学习中文这是很难，因为我有的记性不好坏，我所以常常忘记汉字，我的老师给跟我我说我应该勤恳奋地学习。

大 家 好！