This is a good question. 1 ✔ 这是我的笔。 （笔 may represent one or more, and for the latter you may say 这些是我的笔 as well ） 2 ？ 这是我的一支笔。 3 ？ 这是一支我的笔。 4 ✔ 这支笔是我的。 ( 这笔是我的 is right too) 5 ✘ 这一支笔是我的。 6 ✔ 我要的是这一支（笔），不是那一支（笔）。 （笔 does not usually appear, though） Those number 2, 3 and 5 are ungrammatical or unnatural ( except in special context). The reason is: Generally, when you say CARDINAL NUMBER + MEASURE WORD + NOUN, the noun is indefinite: 我有三支笔，一支笔不够用啊，山坡上有三只羊，... while demonstratives are definite, thus there'd be a contradiction,e.g., so they do not co-occur in a basic phrase; and if the number does not appear, the construction DEMONSTRATIVE + MEASURE WORD + NOUN is alright(4). Specially, if you are showing contrast, 这一支 (vs 那一支）is correct too(6).

The first sentence in English: It's my pen. When people say "my something" doesn't need to use measurement, right? Same in Chinese. The second sentence in English: This pen is mine. When you want to use "This something" in Chinese, don't forget to use measurement.

Measure word is usually to describe the quantity of people, things, or actions which means it's to emphasize the quantity. Sentence 1: 这是我的笔---this one can be considered to emphasize 我的. （ it's my pen, not yours. The focus is the ownership, not the quantity ) Sentence 2: 这只（支）笔是我的---this one can be considered to emphasize 这支。（this one is mine, not that one) ------------------- ***1>However, in most cases , we use all of them without big differences. @这是我的笔--correct! @这支笔是我的=这笔是我的（the 1st one feels much fluent in tone and more literary style, while the 2nd one is a relatively direct expression, oral style, even with a little arguing mood in certain cases)。 ***2> we can also use tone changes to show the key point. If you read 我的 with very heavy tone in both cases, it's to emphasize 我的. ***3> Funny moment among men: C: 这(个)是你（的）老婆吗？ A: 这(个)不是我(的）老婆，是他（的）老婆。 B: 那个老婆是我的， 她们是双胞胎。（we won't say this wife is mine in daily life. It's just for fun to help understand better). C: 噢，原来如此 ----- hope the explanation helps somehow.:D

Good morning! it just describe in more detail so you can also say 这笔是我的.

谢谢你