我学汉语
I'm learning Chinese

我学汉语。

Here is the backbone of the language: subject, then verb, then object — the same order as English. 我 does the studying, 学 is the studying, 汉语 is what gets studied. Compare it with 我是学生 from before: 是 only links two nouns, while 学 is a real action taken on an object. And 学 never changes shape — 我学, 你学, 他学, 我们学 are all just 学, because Chinese verbs have no endings at all.

我不学英语。汉语很难，不容易。

不 goes immediately before the verb, exactly as it did before 是: 不学, 不是, 不忙. Nothing else in the sentence moves, so 我学英语 becomes 我不学英语 and stops there. The adjectives behave the same way: 汉语很难 in the positive, 汉语不难 in the negative. 学, 难 and 容 all carry a rising tone, so 不 keeps its own falling tone in front of them: bù xué, bù nán, bù róngyì — unlike bú lèi and búcuò, where the next syllable falls.

汉语、中文、英语

Two endings name languages. 语 yǔ is the tongue you speak, 文 wén the writing you read, and each attaches to a short name for the people or the country: 汉语 is the language of the 汉 people, 英语 the language of 英国 Britain, 中文 the language of 中国 China. In practice 汉语 and 中文 both mean 'Chinese' and you will hear them used for each other; 汉语 is the usual word in a classroom, 中文 the usual word when the writing is in view.

我学汉语。汉语很难，我很高兴。不错！

Say aloud what you study: 我学汉语. Then say what you do not study: 我不学 …. Then judge it — 汉语很难 or 汉语不难, 英语很容易 or 英语不容易 — and ask a friend back with 你呢？ Four short sentences, and every one of them is subject first, verb second.