一来 … ，二来 … (firstly …, secondly …)

Two good reasons

• A friend asks why you chose to live near your office.
• firstly一来yì láito go to work上班shàng bānconvenient方便fāng biàn，secondly二来èr láirent for a room or house房租fáng zūalso也yěnot不bùexpensive贵guì。 — "Firstly, it's convenient for work; secondly, the rent isn't high."

firstly一来yì lái A，secondly二来èr lái B = firstly …, secondly …

• firstly一来yì lái…，secondly二来èr lái… lists two reasons for a choice, usually in speech.
• Each reason is a short clause; a third can follow with 三来sān lái.

firstly一来yì láiit's cheap便宜pián yi，secondly二来èr láifrom离líhome家jiāclose近jìn。

Explaining a decision

• I / me我wǒ坐地铁zuò dì tiě，firstly一来yì láirapid快kuài，secondly二来èr lái不堵车bù dǔ chē。 — "I take the subway: firstly it's fast, secondly there are no traffic jams."
• 他学tā xuéChinese language中文zhōng wén，firstly一来yì láiis是shìfor为了wèi lework工作gōng zuò，secondly二来èr láiis是shìliking喜欢xǐ huanChina中国zhōng guóculture文化wén huà。 — "He's learning Chinese partly for work and partly because he likes Chinese culture."

firstly一来yì láiis是shìfor为了wèi lework工作gōng zuò，secondly二来èr láiis是shìliking喜欢xǐ huanculture文化wén huà。

Reasons, not steps

Versus on the one hand一方面yì fāng miàn…on the other hand另一方面lìng yì fāng miàn…

• on the one hand一方面yì fāng miàn…on the other hand另一方面lìng yì fāng miàn… weighs two aspects, sometimes in contrast.
• firstly一来yì lái…secondly二来èr lái… simply lists supporting reasons for one decision.

first首先shǒu xiānwash洗xǐthe vegetables菜cài，then然后rán hòucut them切qiē。

Recap

Read the full reference: HSK 6 grammar — Sentence patterns.